disk: Make the disk partition code work with no specific partition types

Currently, if the disk partition code is compiled with all of the parition
types compiled out, it hits an #error which stops the build. This change
adjusts that file so that those functions will fall through to their defaults
in those cases instead of breaking the build. These functions are needed
because other code calls them, and that code is needed because other config
options are overly broad and bring in support we don't need along with
support we do.

Also reduce repetition of the 6-term #ifdef throughout the file.

Signed-off-by: Gabe Black <gabeblack@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>