bootimg: Use STAGING_KERNEL_DIR

bootimg: Use STAGING_KERNEL_DIR

bootimg.bbclass using STAGING_DIR_HOST/kernel instead of
STAGING_KERNEL_DIR, resulting in build failure of live images.

| install: cannot stat `/usr/local/dev/yocto/fishriver-test/build/tmp/sysroots/fishriver/kernel/bzImage': No such file or directory

Replace it with STAGING_KERNEL_DIR.

(From OE-Core rev: 8f16811a8d51982a8b3d70e6087aef4a41926840)

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Tested-by: Tom Zanussi <tom.zanussi@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>