[Skiboot] msglog and dummy console rework

[Oliver O'Halloran]

This series cleans up the skiboot "memcons" which is almost always not
a console. This series does two main things:

1. Splits the current "struct memcons" into a structure that skiboot can
use internally (native endian, not accessible to the BMC) and an external
descriptor structure which follows the existing memcons ABI.

2. It splits the skiboot message log away from the dummy / in-memory
console. This results in all platforms having a similar path for console
output from the OS to the underlying console rather than having a few
wierd ones writing striaght into the skiboot log buffer.