efi_loader: Add logic to parse EDKII specific fmp payload header

When building the capsule using scripts in edk2, a fmp header is
added on top of the binary payload. Add logic to detect presence of
the header. When present, the pointer to the image needs to be
adjusted as per the size of the header to point to the actual binary
payload.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>