linux: add support for Image.gz kernel format

linux: add support for Image.gz kernel format

arm64 / riscv supports building a gzip compressed 'Image' format kernel,
which is sometimes useful. From arch/arm64/Makefile:

all:	Image.gz

Image: vmlinux
	$(Q)$(MAKE) $(build)=$(boot) $(boot)/$@

Image.%: Image
	$(Q)$(MAKE) $(build)=$(boot) $(boot)/$@

(and similar logic for riscv)

Future architectures may or may not copy this logic, so for robustness add
an explicit Image.gz format rather than copying both Image and Image.gz when
the Image format is used.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>