Extend rather than reset memory entries in avrdude.conf

This commit changes the philosophy whenever avrdude.conf encounters the
same memory of a part for the second time or whenever a memory is
described that, through inheritance, already existed: AVRDUDE no longer
zaps the memory, it rather extends it.

Therefore, avrdude.conf.in's entry for ATmega128RFA1, which inherits from
the ATmega2561, needs a line `load_ext_addr = NULL;` in its flash memory
description to zap the inherited load_ext_addr SPI command.

Other than this, avrdude.conf.in needs no other change in order to effect
the same internal representation proving earlier updates to the .conf.in
file correct that manually ensured inheritance of memory contents.

src/avrdude.conf.in

@@ -11845,6 +11845,8 @@ part parent "m2561"
                           " a7   x   x   x      x   x   x   x",
                           "  x   x   x   x      x   x   x   x";
 
+        load_ext_addr   = NULL;
+
 	mode		= 0x41;
 	delay		= 20;
 	blocksize	= 256;