Currently when cmdlinepart_do_parse() is called with an empty
partitions string then an unnamed partition with size 0 is created.
This is wrong of course and instead no partition should be
created.

With this barebox no longer crashes while booting when all partitions
are deleted on the commandline using "nand0.partitions="

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>