missing dependency in building uboot instructions

[Randall Loomis]

Build instructions in README.md at this link:
https://github.com/snapcore/pi3-gadget

Should include installing dtc in the steps to build uboot from source:

sudo apt install gcc-arm-linux-gnueabi
+ sudo apt install device-tree-compiler
export CROSS_COMPILE=arm-linux-gnueabi-
git clone git://git.denx.de/u-boot.git
cd u-boot; git checkout v2017.01-rc1
git apply <gadget-folder>/prebuilt/uboot.patch
make rpi_3_32b_defconfig
make -j8

On base 16.04 system before dtc install step, following error resulted from build:

 DTC arch/arm/dts/bcm2835-rpi-a-plus.dtb
/bin/sh: 1: dtc: not found
scripts/Makefile.lib:299: recipe for target 'arch/arm/dts/bcm2835-rpi-a-plus.dtb' failed
make[2]: *** [arch/arm/dts/bcm2835-rpi-a-plus.dtb] Error 127
dts/Makefile:36: recipe for target 'arch-dtbs' failed
make[1]: *** [arch-dtbs] Error 2
Makefile:1241: recipe for target 'dts' failed
make: *** [dts] Error 2