tools: mkimage: fix STM32 image format for big endian hosts

Two header fields are not properly converted to little endian
before assignment, resulting in incorrect header while executing
mkimage on big endian hosts.

Convert the value of the header fields image_checksum and
edcsa_algorithm to little endian before the assignment.

Signed-off-by: Antonio Borneo <antonio.borneo@st.com>
Reviewed-by: Patrick DELAUNAY <patrick.delaunay@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>