Add a new kernel-pkgs target to create pkg(8) packages from the kernel

Add a new kernel-pkgs target to create pkg(8) packages from the kernel

The version scheme used is the following:
For stable/current branches:
${REVISION}.sYYYYMMDDhhmmss

s standing for snapshot

For releases branches:
${REVISION}_${PATCHLEVEL}

When packaging the kernel 2 different package are created per kernel
release (only contains the regular kernel and modules)
debug (contains the .symbols files)

Note that package the kernel (packaging world will follow the same rule) can
only by passing -DNO_ROOT to the build (hence can be done as a regular user)

To package the kernel:
make buildkernel
make distributekernel DESTDIR=/somewhere -DNO_ROOT
make kernel-pkgs DESTDIR=/somewhere -DNO_ROOT

The packages will be created inside the DESTDIR