Apply a few small optimizations to boot2's code, to make it shrink a

Apply a few small optimizations to boot2's code, to make it shrink a
little further.  This gets us further on the way to be able to build it
successfully with clang.  Using in-tree gcc, this shrinks boot2.bin with
60 bytes, the in-tree clang shaves off 72 bytes, and ToT clang 84 bytes.

Submitted by:	rdivacky
Reviewed by:	imp