Makefile: take default SOURCE_DATE_EPOCH from repo containing Makefile

Makefile: take default SOURCE_DATE_EPOCH from repo containing Makefile

For reproducible builds, SOURCE_DATE_EPOCH will be set to the git commit
date if it is not defined in the environment, but this was done by
explicitly using $(TOPDIR)/.git as the git repository, which would not
give the expected result if Buildroot had been put into a subdirectory
of another repository.

This commit removes that restriction, meaning that the default date will
now be the date of the git commit that contains Makefile, regardless of
what level above Makefile the repository is at. This works because the
current directory when the 'git log' command is executed will always be
the directory containing Makefile (it must be, since TOPDIR is set from
CURDIR).

In general this should be a sensible default, and in cases where a
different date is required SOURCE_DATE_EPOCH can be defined in the
environment before invoking make.

Signed-off-by: James Byrne <james.byrne@origamienergy.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>