binman: ftest: Add test for xilinx-bootgen etype

Add test for the 'xilinx-bootgen' etype

Signed-off-by: Lukas Funke <lukas.funke@weidmueller.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Allow missing bootgen tool; comment testXilinxBootgenMissing() comment:
Signed-off-by: Simon Glass <sjg@chromium.org>