Added missing (dummy) functions attach and detach interrupt to SPIClass (taken from SAM version), and added hooks.c for yield function. Note. use of yield() at the moment isn't possible, because the linker seems unable to find the function even though hooks.c is being compiled
Revert "Move libmaple/* from variants to cores"

This reverts commit a3065b0c0042d5f90d85ed90d88e7aeaccf8aeb9

which resulted in Maple and maple mini boards not running / starting code after upload
Added extern C wrapper around yield()
Added files and made changes to support linux upload
Fixed issue cause by comments in platform.txt on Linux
Changed examples that use SerialUSB to use Serial, also fixed bug associated with use of the reserved word 'index' as a variable name
Added examples provided by Ray aka MrBrunette
Set file permissions on linux/maple_upload to 755