Makefile: Don't require filesystem.cpio.gz if building non-initrd
Makefile: Don't require filesystem.cpio.gz if building non-initrd

Introduce a USE_INITRD switch so we can avoid requiring the user to touch
a pointless filesystem.cpio.gz if they're not using an initrd.

Signed-off-by: Peter Maydell <313afa09ce093b19ae1dd74f57c1ebd94508584c@linaro.org>

virtualopensystems/boot-wrapper,virtualopensystems/boot-wrapper