fix(fwu): rename is_fwu_initialized

The variable is_fwu_initialized was initialized after
plat_fwu_set_images_source() is called.
But some functions called by plat_fwu_set_images_source() for STM32MP1
implementation expect is_fwu_initialized is set to true with asserts.
Rename is_fwu_initialized to is_metadata_initialized, and set it before
plat_fwu_set_images_source() is called.

Change-Id: I17c6ee6293dfa55385b0c859db442647f0bebaed
Signed-off-by: Sebastien Pasdeloup <sebastien.pasdeloup-ext@st.com>
Signed-off-by: Yann Gautier <yann.gautier@st.com>