Master

debug macros for AVR (take #2): Introduce debug macro hooks for AVR

When AVR_HAS_MEMX_PTR is enabled provide dbg macro hooks that place
the format strings in program memory releasing RAM for other tasks.

All AVR toolchain configuration names must end with _TOOLCHAIN to work properly with nuttx/tools/testbuilds.sh

SAMV7: Add logic to check if multiple banks are busy in TXINI interrupt processing.  This logic is probably necessary, but currently commented out because it is untested

AVR: Rename ATMEGA_PC_SIZE to AVR_PC_SIZE since it must be used in common AVR logic.  Add definitions for other AVR architectures as necessary for compilation

Atmega2560

Atmel's AVR8 toolchain is similar to WinAVR, but at present moment
it has one important feature that WinAVR does not. The __flash and __memx
allow built in comiler support for transparent program memory pointers.
Functions that use such pointers will work both with RAM arrays and
flash memory arrays. The compiler generates an appropriate code automatically.
For systems with tiny memory like atmega, this is very important and
allows to move all static const strings into flash.

Atmega2560

The main challenge is to change the context switch code to be aware of the
extra byte that is saved on stack during call and intterupt. This relates also
to the task startup and signal handling.

Put setting of XDIV into ifdef since not AVR platforms support it.