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>

docs/plat/stm32mp1.rst

@@ -153,7 +153,10 @@ To build TF-A BL2 with its STM32 header for SD-card boot:
 .. code:: bash
 
     make CROSS_COMPILE=arm-none-eabi- PLAT=stm32mp1 ARCH=aarch32 ARM_ARCH_MAJOR=7 \
-        DTB_FILE_NAME=stm32mp157c-ev1.dtb
+        DTB_FILE_NAME=stm32mp157c-ev1.dtb STM32MP_SDMMC=1
+
+For other boot devices, you have to replace STM32MP_SDMMC in the previous command
+with the desired devive flag.
 
 This BL2 is independent of the BL32 used (SP_min or OP-TEE)