Validate the TA signed header's img_size
Validate the TA signed header's img_size

Reviewed-by: Jens Wiklander <7706914404370d7502c27a0eff493dcf491feb51@linaro.org>
Suggested-by: Jason Lin <5c1e10b2204269819305f3270afb69755cea76f7@microsoft.com>
Suggested-by: Paul Swan <3ea3ee6cf4d3e2b51e6554881d15b01e9ce42cfa@microsoft.com>
Signed-off-by: Pascal Brand <0b55292d2562685bad4ce52d64f802e8f1cd299e@st.com>

cedric-chaumont-st-dev/optee_os,pascal-brand-st-dev/optee_os,matt2048/optee_os,cedric-chaumont-st-dev/optee_os,pascal-brand-st-dev/optee_os,cedric-chaumont-st-dev/optee_os,pascal-brand-st-dev/optee_os,matt2048/optee_os,cedric-chaumont-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,matt2048/optee_os,matt2048/optee_os