scripts/setlocalversion: sync with linux 5.8

The linux changes since v3.16 are

78283edf2c01 kbuild: setlocalversion: print error to STDERR
b24413180f56 License cleanup: add SPDX GPL-2.0 license identifier to files with no license
6147b1cf1965 scripts/setlocalversion: git: Make -dirty check more robust
8ef14c2c41d9 Revert "scripts/setlocalversion: git: Make -dirty check more robust"
ff64dd485730 scripts/setlocalversion: Improve -dirty check with git-status --no-optional-locks
7a82e3fa28f1 scripts/setlocalversion: clear local variable to make it work for sh
991b78fbd223 scripts: setlocalversion: fix a bashism
3c96bdd0ebfa scripts: setlocalversion: replace backquote to dollar parenthesis

and it's ff64dd485730 that is the motivation for this sync. It fixes
false positive "-dirty" added to the version string when building with
Yocto
(https://lists.openembedded.org/g/openembedded-core/message/137702

).

There has been one U-Boot specific patch since this was synced with
linux 3.16: 81630a3b (scripts: setlocalversion: safely extract
variables from auto.conf using awk).  Keep these changes applied.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
[trini: Re-add 81630a3b and reword message]
Signed-off-by: Tom Rini <trini@konsulko.com>