This doesn't change the behavior of any command, but adds revision.c
as a real source file rather than something created by make using
"echo ... > revision.c".  In any case, the file is recompiled
each time you call "make", so the date and time reflect the last
build.

However, this introduces CONFIG_DETERMINISTIC_BINARY (that nobody
sets at this point in time) to build a binary withouth __DATE__
and __TIME__ strings, so it is the same at every rebuild. See
next commit.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>