module.bbclass: Create a new depmodwrapper to assist cross-installs

Previously the build path to STAGING_KERNEL_DIR was being embedded into the
package post install scripts.  We avoid this behavior by generating a special
depmodwrapper script.  This script contains that hard-coded path, ensuring
that re-use of the sstate-cache (and/or packages) will always run through the
wrapper generated by the current build with a checksum that includes
STAGING_KERNEL_DIR.

[ YOCTO #3962 ]

Signed-off-by: Mark Hatle <mark.hatle@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>