When building a kernel using the O= syntax to save output
files in a separate output directory now create a small Makefile in
that same dir.
This Makefile allow one to use make in the output directory without
the hassle of going back to the kernel source tree.
The O= option is added by this Makefile stub.
Please note that the Makefile silently overwrite an old one, so changes
will be lost if modified.
If there is a need to tweak a Makefile in the output directory it is recommended
to use the filename 'makefile', which GNU Make will try first.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>