lpc23xx has 2k of battery RAM that is retained in Deep Power Down mode.
To not overwrite that data it must only be initialized on Power On Reset.
However, RSIR looks the same when waking up from Deep Power Down as it does
on the power-on case.
So use 4 bytes of the backup RAM to keep a signature that is only valid if
memory was retained (no power-on Reset).
A small change to the linker script is required so two sections can be
placed into flash.
The 10 bit DAC on the lpc23xx is very simple.
It only has one channel and can only be mapped to a single pin (P0.26).
After setting the pin mode to DAC no further configuration in needed.
