MIPS: Fix generic-board-config.sh for builds using O=

MIPS: Fix generic-board-config.sh for builds using O=

When configuring the kernel using one of the generic MIPS defconfig
targets, the generic-board-config.sh script is used to check
requirements listed in board config fragments against a reference config
in order to determine which board config fragments to merge into the
final config.

When specifying O= to configure in a directory other than the kernel
source directory, this generic-board-config.sh script is invoked in the
directory that we are configuring in (ie. the directory that O equals),
and the path to the reference config is relative to the current
directory. The script then changes the current directory to the source
tree, which unfortunately breaks later access to the reference file
since its path is relative to a directory that is no longer the current
working directory. This results in configuration failing with errors
such as:

  $ make ARCH=mips O=tmp 32r2_defconfig
  make[1]: Entering directory '/home/pburton/src/linux/tmp'
  Using ../arch/mips/configs/generic_defconfig as base
  Merging ../arch/mips/configs/generic/32r2.config
  Merging ../arch/mips/configs/generic/eb.config
  grep: ./.config.32r2_defconfig: No such file or directory
  grep: ./.config.32r2_defconfig: No such file or directory
  The base file '.config' does not exist.  Exit.
  make[1]: *** [arch/mips/Makefile:505: 32r2_defconfig] Error 1
  make[1]: Leaving directory '/home/pburton/src/linux-ingenic/tmp'
  make: *** [Makefile:145: sub-make] Error 2

Fix this by avoiding changing the working directory in
generic-board-config.sh, instead using full paths to files under
$(srctree)/ where necessary.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Fixes: 27e0d4b05107 ("MIPS: generic: Allow filtering enabled boards by requirements")
Cc: linux-mips@linux-mips.org
Cc: kbuild test robot <fengguang.wu@intel.com>
Cc: kbuild-all@01.org
Patchwork: https://patchwork.linux-mips.org/patch/17231/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>