For better compatibility copy most of cortexm_base.ld
and use the same section names.
Only interrupt stacks and the two additional (currently unused)
heap sections are different between the two now.
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.
If the .noinit section starts at the beginning of the RAM,
a bootloader that is unaware of it will clear it.
Instead, move it behind the .bss section, hoping that a bootloader
will always use less .bss memory than RIOT proper.
