Allow partial SRAM load

Different cores that emulate the same system may have slightly different
understanding of SRAM, like it currently is for Genesis Plus GX vs
PicoDrive. Currently Genesis Plus GX uses larger padding, which means it
creates larger files. When loading such file for PicoDrive, current
code refuses to load it and this effectively destroys user's save
because new SRAM file is written on PicoDrive's exit.

To fix this, allow partial load (and print a warning).
Merge pull request #337 from notaz/sram

Allow partial SRAM load