XMEM interface support

[r4wabr]

I came across the fact that the environment it is not possible to fully program the AVR of the Mega family with the XMEM interface. I would like to have such an opportunity.

[MCUdude]

I've never used this interface, but it's using avr-gcc underneath, where the user has access to the compiler flags. So if you know how to set up XMEM in, say, a makefile, then it may be possible to port this over to PlatformIO.

[r4wabr]

compiler.ldflags=-Wl,--defsym=__heap_start=0x808000,--defsym=__heap_end=0x80FFFF

[MCUdude]

With PlatformIO it should be able to do:

; Platformio.ini

build_flags = 
  -Wl,--defsym=__heap_start=0x808000,--defsym=__heap_end=0x80FFFF

If that's not working, it looks like you can use an extra_script.