kernel: Add kernel headers to kernel-dev package

[YOCTO #1614]

Add the kernel headers to the kernel-dev package. This packages what was
already built and kept in sysroots for building modules with bitbake.
Making this available on the target requires removing some additional
host binaries.

Move the location to /usr/src/kernel

Before use on the target, the user will need to:

    # cd /usr/src/kernel
    # make scripts

This renders the kernel-misc recipe empty, so remove it.

As we use /usr/src/kernel in several places (and I missed one in the
previous version), add a KERNEL_SRC_DIR variable and use that throughout
the class to avoid update errors in the future.

Now that we package the kernel headers, drop the
kernel_package_preprocess function which removed them from PKGD.

All *-sdk image recipes include dev-pkgs, so the kernel-dev package will
be installed by default on all such images.

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
CC: Bruce Ashfield <bruce.ashfield@windriver.com>
CC: Tom Zanussi <tom.zanussi@intel.com>
CC: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>