package: Add support for kernel stripping

Extend runstrip() to accept additional argument to enable
sharing it with the kernel do_strip() so that
KERNEL_IMAGE_STRIP_EXTRA_SECTIONS can be passed.

Since is_elf() understands kernel modules there is no need to keep a
seperate list for kernmodules or hardcode the values to runstrip.

Signed-off-by: Saul Wold <saul.wold@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>