Makefile: Use sort shortopts

POSIX does not defined longopts for sort, use shortops
for even more compatibility.

Fixes: cc5a490c ("Makefile: Sort u-boot-initial-env output")
Reported-by: Milan P. Stanić <mps@arvanta.net>
Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Mark Kettenis <kettenis@openbsd.org>
Tested-by: Milan P. Stanić <mps@arvanta.net>