lib/flash: Add support for platform versions

Add basic libflash support to read the VERSION partition on BMC
machines. This adds a dependency on the libflash shared library from
Skiboot.
The MTD partition is accessed through the libflash helper functions.
Once read into a buffer, the VERSION partition is a simple list of
newline-terminated version strings. Some BMC platforms may have two
'sides' to flash - these are interpreted as the "current" and "other"
sides depending on which one the current Petitboot is running from.

Signed-off-by: Sam Mendoza-Jonas <sam@mendozajonas.com>