Avoid hardcoding of LED, Button and USB disconnect banks and pins

Also remove spurious configuration of the whole LED (GPIOA) bank.

Signed-off-by: Tormod Volden <debian.tormod@gmail.com>
Allow passing PLATFORM to the build to specify target board type

Example:
make PLATFORM=MAPLE

Signed-off-by: Tormod Volden <debian.tormod@gmail.com>
Add MAPLE_MINI platform build option

Example:
make PLATFORM=MAPLE_MINI

Signed-off-by: Tormod Volden <debian.tormod@gmail.com>