pic24f: Make the standard linker script more generic

Move the memory map into pic24f_mem.txt and move CRTn_STARTUP
items to MCU-specific files. Modify the generator scripts to look
for the new pic24f_mem.txt file.

Note that there are renames here, so use git log --follow to track.
	pic24fj64gb002_mem.txt -> pic24f_mem.txt
	pic24fj64gb002_mem_bootloader.txt -> pic24fj64gb002_mem.txt
pic24f: Add pic24fj256da206 and pic24fj256da210
pic24f: Don't write the last flash block in the bootloader configuration

The last flash block contains the configuration registers. Unfortunately,
they get erased when the last flash block is cleared.  This means we
can't let the bootloader code write anything into this last erase block.