ath25: fix initramfs image generation

ath25: fix initramfs image generation

Commit 21f460a5dbef ("ath25: fix duplicate LZMA compression") changed
the way kernel images are generated, affecting initramfs images instead.
Initramfs images were previously ELF images, and by mistake this change
caused the raw kernel image to be used as a source. This caused them to
be non-loadable by bootloaders.

Restore the previous KERNEL_INITRAMFS recipe and adjust
KERNEL_INITRAMFS_NAME to point at the correct source artifact.
While at that, adjust KERNEL_INITRAMFS_SUFFIX to -kernel.elf,
so it matches the suffix of non-initramfs kernel artifact.

Fixes: 21f460a5dbef ("ath25: fix duplicate LZMA compression")
Signed-off-by: Lech Perczak <lech.perczak@gmail.com>