In build_arm_none_eabi_gcc_m0plus, use baremetal_size instead of baremetal
as the configuration, i.e. exclude debugging features. This job is the only
one switching to baremetal_size because it's our primary point of reference
for code size evolution, and which is the only job where we display the code
size built with -Os so it's presumably the only job for which we really care
about a meaningful code size report.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>