Makefile: avoid executing 'git log' each time SOURCE_DATE_EPOCH is used

If SOURCE_DATE_EPOCH is not defined it was given a definition that
caused 'git log' to be executed each time the variable is referenced,
which is not very efficient given that the answer cannot change.

This commit moves the definition of BR2_VERSION_GIT_EPOCH after the
inclusion of Makefile.in (so that GIT is defined) and makes it a
simply expanded variable so that it is only evaluated once.

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>