This consolidates some of the work done for TARGET_NO_NVMEM and feat/no-direct-memmap. If nvmem is disabled at compile time or alloc fails _nvmem_enabled() will return false. Various other fixes and cleanups all around.
Conflicts: core/hw/pvr/spg.cpp core/hw/sh4/interpr/sh4_interpreter.cpp reicast/android/src/com/reicast/emulator/MainActivity.java