The function would always return `true` after early boot, so it
is not very useful for applications.
Now it will only (but always) return true when we woke from Deep Sleep
*after* early boot. This makes it behave the same ways as the function
of the same name on SAME54.
Rename the existing function to cpu_backup_ram_is_initialized() to better
match it's semantics.
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.
