Add quick config.

This is designed for quick compiling when developing.
No modules are needed and it includes all Pi specific drivers
VCHIQ: Use unsigned integers for unsigned values

The VCHIQ interface used signed integers for values which are
inherently unsigned. Worse, the parameter validation code treated them
as if they were unsigned, checking for overflow but not underflow.

This patch converts those integers to unsigned integers.

Thanks to Jann Horn.
mmc: suppress sdcard warnings we are happy about by default
Allow reboot=q on command line to set a flag that bootcode.bin can use to boot from alternate partition