Removed u-boot binaries
Added the source for the RPi port of U-Boot

This is not yet integrated into the Xinu build process,

U-boot can be built by:
1. CROSS_COMPILE=arm-none-eabi-
2. export CROSS_COMPILE
3. make rpi_b
The output file of interest is u-boot.bin

The u-boot script can then be built using mkimage which was also compiled in the previous step.

./tools/mkimage -A arm -O linux -T script -C none -d tftp-boot.cmd boot.scr