docs: stm32mp1: update BL2 compilation command

Add the boot device flag in the compilation command line.
And add a comment about boot devices flags.

Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
Change-Id: I105b8907e584d3fcb73aa740cd170ae10f067451
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/198703


Reviewed-by: CITOOLS <MDG-smet-aci-reviews@list.st.com>
Reviewed-by: CIBUILD <MDG-smet-aci-builds@list.st.com>
Reviewed-by: Christophe KERELLO <christophe.kerello@st.com>