16-bits is not large enough if using RAMPZ

The RAMPZ register allows the effective address to be up to 24-bit,
so a 16-bit variable is not large enough to hold it.
megax8: Add missing define before including AVR hdr

mega48, mega88, and mega168 all share a common header and rely on their
platform to be defined prior to their headers being included.
flash: add support for RWWSRE bit in SPMCR

The Self Program Memory Control Register (SPMCR) has a bit for enabling
the read while write section (RWWSRE). This is also used for clearing
the temporary page buffer that is used for page writing (PGWRT).

This commit does not wire up the behavior but just allows the flash
structure to understand that it supports a RWW section.
flash: Add support for TEMPPAGE writing.

The proper way to write to flash on the AVR is to write
to the temp page and then write that temp page to flash.
This allows cancellation with the RWWSRE bit for clearing
out the temp page without writing it to flash.

This fixed a bug where the RWWSRE bit was being set in a
command, but the flash driver was improperly writing whatever
was in r0/r1 to the first address in the page, overwriting
what was already there. In reality, the code running on the
core was trying to just flush the temp page or unlock the read
while write section.
Merge branch 'flash-fix' into merged