doc: stm32mp1: Update build command line

Add new flags for storage support that must be used in the build
command line. Add the complete build steps for an OP-TEE configuration.

Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Change-Id: I0c682f6eb0aab83aa929f4ba734d3151c264aeed
1 file changed