build: fix incorrect initramfs bzip2 compression

build: fix incorrect initramfs bzip2 compression

Requires: tools/bzip2

fix consistency of executable to use `$(STAGING_DIR_HOST)/bin/bzip2`, and
not system-installed ones from the usual environment `PATH`;
this affects option `CONFIG_KERNEL_INITRAMFS_COMPRESSION_BZIP2`

this may have worked in the past but only via side effect of having the
binaries on the host system (and whatever unpredictable version or
patchset those might be)

Fixes: 330bd380e8b6 ("image: allow building FIT and uImage with ramdisk")
Signed-off-by: Tony Butler <spudz76@gmail.com>