ptest.bbclass: Handle the case when Makefile does not exist in do_install_ptest_base

ptest.bbclass: Handle the case when Makefile does not exist in do_install_ptest_base

Some recipes e.g. expant generate ninja files and no Makefile may exist,
therefore grep -q would fail on a non-existent file, we just need the
return code  to decide if intall-ptest target should be run or not.

Fixes errors like
| grep: Makefile: No such file or directory

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>