STM32L0 chips use loader_code_stm32lx, but this flash loader is built
for armv7-m and uses instructions that are unsupported on STM32L0 (which
have Cortex M0+ cores implementing armv6-m).
In particular, loader_code_stm32lx uses variants of add-immediate that
do not update the condition flags ( `add r0, r0, #4` ). These are 32bit
instructions in armv7-m and are not available in armv6-m.
Enable loader_code_stm32lx to run on both armv6-m and armv7-m by
building for armv6-m, which requires changing the `add` instructions to
`adds` instructions that do update condition flags (which is ok because
the subs updates the condition flags again before the branch).