MFC r330929: pkgbase: Fix post-install script for kernel packages

kernel.ucl uses a hardcoded boot/kernel for kldxref, which is the incorrect
directory when we're installing extra kernels that aren't the "default"
kernel (placed at boot/kernel).

Fix this by instead using a new %KERNELDIR% that we now replace in
Makefile.inc1 with "kernel" for the default kernel and "kernel.${_kernel}"
for these extra kernels so that, e.g. /boot/kernel.SHIVA, will get properly
kldxref'd upon update and avoid outdated linker.hints.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Merge the projects/release-pkg branch to head.

This allows packaging the base system with pkg(8), including
but not limited to providing the ability to provide upstream
binary update possibilities for non-tier-1 architectures.

This merge is a requirement of the 11.0-RELEASE, and as such,
thank you to everyone that has tested the project branch.

Documentation in build(7) etc. is still somewhat sparse, but
updates to those parts will follow.

Sponsored by: The FreeBSD Foundation

Add FreeBSD RCSID header and svn:keyword property.

Sponsored by: The FreeBSD Foundation

Prefer https in package URLs.

Sponsored by: The FreeBSD Foundation

Fix script syntax

Make scripts cross installation friendly

Now that pkg create can directly accept both a file and a plist in command line,
directly use it

Add a packages/ subdirectory which will contain the metadata for packaging base
Add a first example to package the kernel