Retroleap v2.0.0-alpha2

Another year, another milestone on the road to v2 of Retroleap.

alpha2 primarily improves LF1000 support, including port of gmenunx (gmenu2x fork), and non-retroarch emulator cores that run fast enough to be usable on the LF1000 (Explorer/Didj) hardware.

-For Didj, you'll probably need to change your bootloader from lightningboot to emeraldboot. For more info, see here: https://elinux.org/Didj_Emerald_Boot - note that flashing the bootloader incorrectly can brick your didj, though it should be recoverable with some effort and bridging of pins + UART boot.

-For Leapster Explorer, things should work out of the box.

-LeapPad 1 is not yet tested/supported.

Major changes:
-Add gmenunx to build system
-Add picodrive, ohboy, and pocketSNES to LF1000 version
-(Incomplete) support for Didj-Fi wifi cartridge (https://wiki.jkent.net/devices/pollux/didj-fi?redirect=1)

Known issues;
-Performance regressions on LF2000, probably due to addition of SD card polling
-PocketSNES does not run on didj port (likely due to insufficient RAM but have not investigated)
-Volume buttons don't work on LF1000
-Brightness control doesn't work on LF1000
-LX kernel config does not match tree
-Placeholder image for gmenunx background

Things are still very much a work in progress for 2.0, but welcome any feedback, PRs etc.