cmdlinepart: add function to parse a cmdline partition string ...

This adds a function to parse a full cmdline partition string. The
addpart command is switched to use this function.

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

cmdlinepart: make argument types safer ...

The return size should be loff_t to support partitions bigger
than 4G. Also use const for the devname and endp.

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

move cmdline partition parsing code to separate file ...

So it's no longer local to the addpart/delpart code and can be used
from other code.

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