The callsites to decode_block and decode_partition in the body of
decode_partition are rather noisy with code that is enabled or
disabled by config flags, so it's hard to see what is fundamentally a
simple recursion.

This patch moves all that cruft into a pair of locally defined
preprocessor macros (DEC_BLOCK and DEC_PARTITION) to simplify
things.

There's no diff in disassembled code when compiling before and after
the patch.

Change-Id: Ie08db263a9874e2c566c758e8f3a51d131b1b80b