binman: btool: Add Xilinx Bootgen btool

Add the Xilinx Bootgen as bintool. Xilinx Bootgen is used to create
bootable SPL (FSBL in Xilinx terms) images for Zynq/ZynqMP devices. The
btool creates a signed version of the SPL. Additionally to signing the
key source for the decryption engine can be passend to the boot image.

Signed-off-by: Lukas Funke <lukas.funke@weidmueller.com>