kbuild: eradicate bashisms in scripts/patch-kernel

Make the patch-kernel shell script sufficiently compatible with POSIX
shells, i.e., remove bashisms from scripts/patch-kernel.
This means that it now also works on dash 0.5.3-5
and still works on bash 3.1dfsg-8.

Full changelog:
- replaced non-standard "==" by standard "="
- replaced non-standard "source" statement by POSIX "dot" command
- use leading ./ on mktemp filename to force the tempfile to a local
  directory, so that the search path is not used
- replace bash syntax to remove leading dot by similar POSIX syntax
- added missing (optional/not required) $ signs to shell variable names

Signed-off-by: Andreas Mohr <andi@lisas.de>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>