android: boot: replace android_image_check_header

android: boot: replace android_image_check_header

With the new vendor boot image introduced in versions 3 and 4
of boot image header, the header check must be done for both boot
image and vendor boot image. Thus, replace android_image_check_header()
by is_android_boot_image_header() to only refer to boot image header check.

Signed-off-by: Safae Ouajih <souajih@baylibre.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Tested-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>