ARMv7: Rewrote serial drivers on ARM.

ARMv7: Rewrote serial drivers on ARM.

They're now fairly well compartmentalised on both ARM and x86, and can start
automatically from Kaluga.  There's also a 'kernel' serial driver, which uses
the syscall interface to read and write on the kernel console.  This avoids
having user level and kernel code racing on access to the UART.  Fish works
(minus line editing) on all Versatile Express platforms.

Signed-off-by: David Cock <david.cock@inf.ethz.ch>