ptest: Reproducibility: Strip build host paths from any installed Makefile

ptest: Reproducibility: Strip build host paths from any installed Makefile

A common pattern for ptest is to install a Makefile that provides a make check
target. These generated Makefiles are normally full of build host paths, to
hosttools, and to files and scripts within the source or build tree. They also
commonly include the CFLAGS and CPPFLAGS. None of these build host paths
can possibly work on-target, so stripping them has no effect on the success of
the resulting ptests.

Stripping the HOSTTOOLS_DIR has this effect, for instance:

    -MKDIR_P = /home/douglas/workspace/upstream/build/tmp/hosttools/mkdir -p
    +MKDIR_P = mkdir -p

The curious WORKDIR/*= pattern avoids non-parseable -fdebug-prefix-map patterns
in the CFLAGS by adding an arbitrary ".", eg:

    -fdebug-prefix-map=.=/usr/src/debug/rsyslog/8.37.0-r0

In all other cases, we simply remove the WORKDIR, eg:

    CPP = arm-tait-linux-gnueabi-gcc -E --sysroot=recipe-sysroot ...

Signed-off-by: Douglas Royds <douglas.royds@taitradio.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>