refactor(stm32mp1): split compilations per boot devices

To reduce BL2 size and leave bigger room for BL32, BL2 is compiled
for a given boot device. The default config keeps SD-card and eMMC,
as they use the same MMC framework.
Add the required files to tf-l1-build-plat.
Remove bash line from group/tf-l1-build-plat/stm32mp1-default:nil.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I0f5fddabcfde34b156915c2d0d375bd1b7d2a889
8 files changed