kbuild: sync with Linux 4.2

Update some build scripts to match Linux 4.2.  Commit-based syncing was
done so as to not break U-boot specific changes.  The previous big sync
was from Linux 4.1 by commit 98bd0e0d (kbuild: sync with Linux 4.1).

The commits imported from Linux (some with adjustments) are:

[1] commit 61754c18752ffb78145671e94f053fb202fff041
    Author: Michal Marek <mmarek@suse.cz>
    kbuild: Allow arch Makefiles to override {cpp,ld,c}flags

[2] commit 6dd3f13e4239a8c2b1e60687d7321bc0df614f33
    Author: Michal Marek <mmarek@suse.com>
    kbuild: Do not pick up ARCH_{CPP,A,C}FLAGS from the environment

[For all of the Kbuild changes within Linux]
Signed-off-by: Michal Marek <mmarek@suse.com>

[For applying within U-Boot]
Signed-off-by: Tom Rini <trini@konsulko.com>