Also use this opportunity to cleanup support for multiple devices: the shell
now supports dynamic changes in the device and will call init() everytime
a new device is selected, to prepare a new environment. The shell now
honors register width on register read/write. The shell also provides access
to variants as follows by creating a subtable under the register using the
variant type in UPPER case and having the same layout as a register.
For example if register HW.GPIO.DIR has variants "set" and "clr", those can
be used like this:
HW.GPIO.DIR.SET.write(0xff)
HW.GPIO.DIR.CLR.write(0xff00)
Change-Id: I943947fa98bce875de0cba4338e8b7196a4c1165