linux: add support for ELF vmlinux image

The ELF vmlinux image found at the root of the kernel source tree is
the format that Qemu needs when emulating mips(el) or ppc targets, so
add support for it.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>