Provide cached byte-wise read/write API (#1106)

* Provide cached byte-wise read/write API

int avr_read_byte_cached(const PROGRAMMER *pgm, const AVRPART *p, const
  AVRMEM *mem, unsigned long addr, unsigned char *value);

int avr_write_byte_cached(const PROGRAMMER *pgm, const AVRPART *p, const
 AVRMEM *mem, unsigned long addr, unsigned char data);

int avr_flush_cache(const PROGRAMMER *pgm, const AVRPART *p);

int avr_chip_erase_cached(const PROGRAMMER *pgm, const AVRPART *p);

int avr_reset_cache(const PROGRAMMER *pgm, const AVRPART *p);

avr_read_byte_cached() and avr_write_byte_cached() use a cache if paged
routines are available and if the device memory is EEPROM or flash,
otherwise they fall back to pgm->read_byte() and pgm->write_byte(),
respectively. Byte-wise cached read always gets its data from the cache,
possibly after reading a page from the device memory. Byte-wise cached
write with an address in memory range only ever modifies the cache. Any
modifications are written to the device after calling avr_flush_cache() or
when attempting to read or write from a location outside the address range
of the device memory.

avr_flush_cache() synchronises pending writes to EEPROM and flash with the
device. With some programmer and part combinations, flash (and sometimes
EEPROM, too) looks like a NOR memory, ie, one can only write 0 bits, not 1
bits. When this is detected, either page erase is deployed (eg, with parts
that have PDI/UPDI interfaces), or if that is not available, both EEPROM
and flash caches are fully read in, a pgm->chip_erase() command is issued
and both EEPROM and flash are written back to the device. Hence, it can
take minutes to ensure that a single previously cleared bit is set and,
therefore, this routine should be called sparingly.

avr_chip_erase_cached() erases the chip and discards pending writes() to
flash or EEPROM. It presets the flash cache to all 0xff alleviating the
need to read from the device flash. However, if the programmer serves
bootloaders (pgm->prog_modes & PM_SPM) then the flash cache is reset
instead, necessitating flash memory be fetched from the device on first
read; the reason for this is that bootloaders emulate chip erase and they
won't overwrite themselves (some bootloaders, eg, optiboot ignore chip
erase commands altogether) making it truly unknowable what the flash
contents on device is after a chip erase. 

For EEPROM avr_chip_erase_cached() concludes that it has been deleted if a
previously cached EEPROM page that contained cleared bits now no longer
has these clear bits on the device. Only with this evidence is the EEPROM
cache preset to all 0xff otherwise the cache discards all pending writes
to EEPROM and is left unchanged otherwise.

Finally, avr_reset_cache() resets the cache without synchronising pending
writes() to the device.

src/CMakeLists.txt

@@ -139,6 +139,7 @@ add_library(libavrdude
     avr.c
     avr910.c
     avr910.h
+    avrcache.c
     avrdude.h
     avrftdi.c
     avrftdi.h