OPTEE-OS-STM32MP: update archiver to manage dt config

When provided device tree is not one of the embedded one in official source
code, we need to set the right configuration switch:
CFG_STM32MP13=1 or CFG_STM32MP15=y
And also we need to manage the DRAM size to configure.

Change-Id: I7d3dc67b89ce2ff1c6330d3ce1b07a5e4344eb61
Signed-off-by: Romuald JEANNE <romuald.jeanne@st.com>

recipes-security/optee/optee-os/README.HOW_TO.txt

@@ -104,10 +104,15 @@ Since OpenSTLinux activates FIP by default, FIP_artifacts directory path must be
   - In case of using SOURCES-xxxx.tar.gz of Developer package the FIP_DEPLOYDIR_ROOT must be set as below:
     $> export FIP_DEPLOYDIR_ROOT=$PWD/../../FIP_artifacts
 
-To compile optee-os source code
+To compile optee-os source code with default embedded config:
     $> make -f $PWD/../Makefile.sdk all
-or for a specific config :
-    $ make -f $PWD/../Makefile.sdk CFG_EMBED_DTB_SOURCE_FILE=stm32mp157c-ev1 all
+You can check the configuration throught 'help' target:
+    $ make -f $PWD/../Makefile.sdk help
+
+To compile only one of the provided devicetree:
+    $ make -f $PWD/../Makefile.sdk CFG_EMBED_DTB_SOURCE_FILE=stm32mp157f-ev1 all
+For a specific devicetree file you need to force not only the devicetree but also the DRAM size settings:
+    $ make -f $PWD/../Makefile.sdk CFG_EMBED_DTB_SOURCE_FILE=<your_devicetree> OPTEE_DRAMSIZE=<dram_size_value_in_hexadecimal> all
 
 By default, the build results for this component are available in $PWD/../deploy directory.
 If needed, this deploy directory can be specified by added "DEPLOYDIR=<your_deploy_dir_path>" compilation option to the build command line above.