I tried reproducing this so I took my board and the image referenced above. I flashed it to an 8GB card, ensured that the boot select DPI switches were set up for booting from SD card (switch #2 on, rest off) and hit the power button.

To my surprise I booted to the eMMC image I happened to have. I tried several times but I was unable to boot the SD card at all.

Not sure if this helps but those are the early boot loader messages:

Format: Log Type - Time(microsec) - Message - Optional Info
Log Type: B - Since Boot(Power On Reset),  D - Delta,  S - Statistic
S - QC_IMAGE_VERSION_STRING=BOOT.BF.3.0-00286
S - IMAGE_VARIANT_STRING=HAAAANAZA
S - OEM_IMAGE_VERSION_STRING=CRM
S - Boot Config, 0x000002e3
S - Core 0 Frequency, 0 MHz
B -      1545 - PBL, Start
B -      3490 - bootable_media_detect_entry, Start
B -    521579 - bootable_media_detect_success, Start
B -    521583 - elf_loader_entry, Start
B -    523171 - auth_hash_seg_entry, Start
B -    523382 - auth_hash_seg_exit, Start
B -    539271 - elf_segs_hash_verify_entry, Start
B -    600304 - PBL, End
B -    476867 - SBL1, Start
B -    526155 - pm_device_init, Start
D -     14182 - pm_device_init, Delta
B -    540673 - boot_flash_init, Start
D -         0 - boot_flash_init, Delta
B -    544699 - boot_config_data_table_init, Start
D -   5106706 - boot_config_data_table_init, Delta - (452 Bytes)
B -   5655950 - CDT version:3,Platform ID:24,Major ID:1,Minor ID:0,Subtype:0
B -   5662203 - sbl1_ddr_set_params, Start
B -   5666015 - cpr_init, Start
D -         0 - cpr_init, Delta
B -   5671627 - Pre_DDR_clock_init, Start
D -       213 - Pre_DDR_clock_init, Delta
D -         0 - sbl1_ddr_set_params, Delta
B -   5684132 - pm_driver_init, Start
D -      3477 - pm_driver_init, Delta
B -   5696332 - SBC platform detected: XO_ADJ_FINE = 0x20
B -   5696363 - clock_init, Start
D -        30 - clock_init, Delta
B -   5711277 - Image Load, Start
D -     27999 - QSEE Image Loaded, Delta - (567468 Bytes)
B -   5739307 - Image Load, Start
D -       427 - SEC Image Loaded, Delta - (2048 Bytes)
B -   5746535 - sbl1_efs_handle_cookies, Start
D -        61 - sbl1_efs_handle_cookies, Delta
B -   5754313 - Image Load, Start
D -     13725 - QHEE Image Loaded, Delta - (56048 Bytes)
B -   5768068 - Image Load, Start
D -     13481 - RPM Image Loaded, Delta - (149316 Bytes)
B -   5781580 - Image Load, Start
D -     28822 - APPSBL Image Loaded, Delta - (519844 Bytes)
B -   5810433 - QSEE Execution, Start
D -        61 - QSEE Execution, Delta
B -   5816136 - SBL1, End
D -   5341618 - SBL1, Delta
S - Flash Throughput, 101000 KB/s  (1295176 Bytes,  12749 us)
S - DDR Frequency, 400 MHz
Android Bootloader - UART_DM Initialized!!!
[0] [0] BUILD_VERSION=dragonboard410c-LA_BR_1_2_7-03810-8x16_0-linaro2
[0] [0] BUILD_DATE=15:13:18 - Feb  5 2018
[0] [0] welcome to lk

[10] [10] platform_init()
[10] [10] target_init()
[40] [40] SDHC Running in HS200 mode
[90] [90] Done initialization of the card
[90] [90] pm8x41_get_is_cold_boot: cold boot
[100] [100] Neither 'config' nor 'frp' partition found
[100] [100] Not able to search the panel:
[110] [110] Display not enabled for 24 HW type
[110] [110] Target panel init not found!
[120] [120] pm8x41_get_is_cold_boot: cold boot
[120] [120] partition misc doesn't exist
[120] [120] error in emmc_recovery_init
[130] [130] Unable to locate /bootselect partition
[130] [130] No 'misc' partition found
[140] [140] Error reading MISC partition
[140] [140] failed to get ffbm cookie[150] [150] Loading boot image (11960320): start
[220] [220] Loading boot image (11960320): done
[230] [230] use_signed_kernel=0, is_unlocked=0, is_tampered=0.
[670] [670] qcom,msm-id entry not found
[670] [670] Only one appended non-skales DTB, select it.
[680] [680] target_display_panel_node:510: hw_id=24 panel_name=""
[680] [680] cmdline: root=/dev/disk/by-partlabel/rootfs rw rootwait console=tty0 console=ttyMSM0,115200n8 androidboot.emmc=true androidboot.serialno=bf4c2530 androidboot.baseband=apq mdss_mdp.panel=0:dsi:0:
[700] [700] Updating device tree: start
[790] [790] Setting WLAN mac address in DT: 02:00:BF:4C:25:30
[810] [810] Setting Bluetooth BD address in DT: 02:00:BF:4C:25:31
[810] [810] Setting BT mac address in DT: 02:00:BF:4C:25:31
[820] [820] Updating device tree: done
[830] [830] booting linux @ 0x80080000, ramdisk @ 0x84000000 (4004777), tags/device tree @ 0x83e00000
[830] [830] Jumping to kernel via monitor

After that the Linaro kernel I happened to have on the eMMC continued to boot normally.