Increasing MIPS emulator system image size and user-data image size

Size increase taken as a precaution against recent built breakage
due to lack of space on a number of targets (e.g. x86, ARM64).
System and user-data image sizes set to match currently most common
setup for the emulator: system image: 1.25gb, user-data image 700mb.

Change-Id: I7118eb26dd78f5fa9e4f0006e15c8d47dee8e28c