This fixes:
0a37e22d (bootm: use names instead of numbers for image parts)

This commit switches to strings for the image numbers for better FIT
image support (which uses names instead of numbers). These strings
may be NULL when no image number is given. They are used uninitialzed
in several places. Introduce a wrapper function to convert the string
into a number. Check for NULL here in which case we return 0 which
is the correct value.

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