Changes

gcc/Makefile: set shell: bash -o pipefail -e

Don't ignore errors from commands piped into others, like in this line
in the Makefile that has been added in aaec8734 ("add fw update
crcstub"):

  printf $$CRC_HEX | xxd -r -p | xxd -e | xxd -r | dd of=$(OUTPUT_FILE_NAME)-dfu.bin bs=1 seek=$$CRC_CHECKSUM_OFFSET conv=notrunc 2>/dev/null; \

Change-Id: I9459f63828eab461607fa9f226716d32ae898549

gcc/Makefile: fail early if xxd is not installed

Since aaec8734 ("add fw update crcstub") the xxd program is required for
running the Makefile.

Change-Id: I6a3e7021929ba58c14f1ffced5702d2dfa839ebe