scripts/requirements: add lpc_checksum script

On some NXP LPC MCUs (such as LPC11U6x), a valid firmware image must
include a checksum in the 7th entry of the ARM Cortex-M0 exception
vector table. This checksum is verified by the bootloader.

The lpc_checksum.py python script allows to insert this checksum into
the firmware images (BIN and HEX formats).

Project page: https://github.com/basilfx/lpc_checksum



Signed-off-by: Simon Guinot <simon.guinot@seagate.com>