testing/gxlimg: new aport

gxlimg is a boot image creation tool for Amlogic's GXL and G12A/G12B
SoCs, which replaces the proprietary aml_encrypt binary utilities.  It is
used for creating a boot image with Amlogic firmware and U-Boot,
suitable for booting on Amlogic boards.

Signed-off-by: Ferass El Hafidi <funderscore@postmarketos.org>