ti85: use direct update handler to remove boot memory mapping hacks

This allows the flash calculators to properly reset pc to 0x0000 instead
of 0x8000 and properly reset all memory banks to page 0x00.
Merge pull request #22 from Jonimoose/ti85

ti85: use direct update handler to remove boot memory mapping hacks

out of whatsnew:
This allows the flash calculators to properly reset pc to 0x0000 instead
 of 0x8000 and properly reset all memory banks to page 0x00.

This is based on some example code Judge. Possibly performance improvement could be gained by having the callback unregister itself and register on reset.