io: io_stm32image: add a header size variable

A variable hdr_sz is created in stm32image_partition_read() function.
It just represents the size of the stm32 image header but it really
increases the readability of the function.

Change-Id: I95ec62a78a4b6c6a75b0d8c8aa0faef8bee424da
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/170469


Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: Etienne CARRIERE <etienne.carriere@st.com>