Add support for FIT images to rpi config

[Dave Jones]

Support for signed FIT images has been requested; the following config items should be enabled for the rpi 3 and 4 configurations (in both armhf and arm64):

CONFIG_MULTI_DTB_FIT_UNCOMPRESS_SZ=0x8000
CONFIG_FIT=y
CONFIG_FIT_EXTERNAL_OFFSET=0x0
CONFIG_FIT_FULL_CHECK=y
CONFIG_FIT_VERBOSE=y
CONFIG_FIT_PRINT=y
CONFIG_TOOLS_FIT=y
CONFIG_TOOLS_FIT_FULL_CHECK=y
CONFIG_TOOLS_FIT_PRINT=y
CONFIG_TOOLS_FIT_RSASSA_PSS=y
CONFIG_TOOLS_FIT_SIGNATURE=y
CONFIG_TOOLS_FIT_SIGNATURE_MAX_SIZE=0x10000000
CONFIG_TOOLS_FIT_VERBOSE=y
CONFIG_ZSTD=y

Check this doesn't affect core images, and focal on classic (there will inevitably be a size increase as a result of enabling more flags; this is probably a good time to unseed u-boot on the pi images for noble).