Build: Update git.mk to support R/O src-dir
Build: Update git.mk to support R/O src-dir

With out-of-tree builds with read only source directory the
generation of .gitignore failed the build.
Update it to issue a warning and continue.
Taken from upstream.

Signed-off-by: Andreas Fuchs <622ac2c25087201462d23b927a044354eba8dded@sit.fraunhofer.de>

01org/tpm2-tss,tpm2-software/tpm2-tss,tpm2-software/tpm2-tss,01org/tpm2-tss,01org/tpm2-tss,tpm2-software/tpm2-tss