Rockbox

In anticipation of the new sandisk sansa clip zip players, this is a proposal for the dualboot part of the bootloader.

I propose three steps:
1) Use the same kind of dualboot as for all other AMSv2 players, but make it always boot to the OF. This step proves that our dualboot mechanism with decompression works at all
2) Detect if a specific button is pressed and add a boot delay if it is pressed. This step proves whether we can read a button reliably. I chose the button on GPIO A6 (my guess is that it’s one of the volume buttons), because this is a button that is read out directly in the OF and does not rely on key matrixing. If the button readout turns out to not work reliably, the firmware still boots to the OF.
3) Implement the full dualboot code, with the button press deciding between booting either the OF or the rockbox bootloader.

Please comment and/or give suggestions for improvement.

One thing I worry about a bit in the current dualboot code, is the lack of instructions to switch the GPIO pins to the GPIO function and to switch them as GPIO input.