flash: add virtual flash bank driver

flash: add virtual flash bank driver

This adds a virtual flash bank driver that allows virtual banks to
be defined that refer to an existing flash bank.

For example the real address for bank0 on the pic32 is 0x1fc00000
but the user program will either be in kseg0 (0xbfc00000) or
kseg1 (0x9fc00000).
This also means that gdb will be aware of all the read only flash
addresses.

Signed-off-by: Spencer Oliver <ntfreak@users.sourceforge.net>