Kbuild: Pass absolute path to $(shell) commands

The PWD of the Kbuild script is the current source directory, whereas
the PWD of these $(shell) exectutables is the output directory, which is
usually different to keep the source directory clean.

Signed-off-by: Marijn Suijten <marijns95@gmail.com>
Change-Id: Icd8e445ecfd0dace36aca17e67782f01838d776f