arm-v7m: update linker script for the removing of dead code
arm-v7m: update linker script for the removing of dead code

Update linker to merge subsection, i.e. merge .X.Y into section .X, and add
KEEP keyword for section that we want to keep (e.g. boot code).

Signed-off-by: Fabien Parent <2a93cde040adee823a667eed2882d05f12a84573@gmail.com>

fabo-corp/phabos,fabo-corp/phabos,gregkh/phabos,gregkh/phabos,mtitinger/phabos,mtitinger/phabos