uboot-imx

Makefile: fix SOURCE_DATE_EPOCH for *BSD host

The SOURCE_DATE_EPOCH mechanism for reproducible builds require some date(1)
with -d switch to print the relevant date and time strings of another point of
time.

In other words it requires some date(1) that behaves like the GNU date(1) [1].
The BSD date(1) [2] on the other hand has the same switch but with a different
meaning.

Respect this and check the date(1) abilities before usage, error on non
working version.  Use the well known pre- and suffixes for the GNU variant of
a tool on *BSD hosts to search for a working date(1) version.

[1] http://man7.org/linux/man-pages/man1/date.1.html [2]
http://www.freebsd.org/cgi/man.cgi?query=date

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>