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.

For AVR chips (Atmega2560) Program Counter register larger than 16 bits
EIND represents the most significant byte that is used for EICALL and EIJMP
instructions.

Setting of EIND is normally managed by compiler, but I have seen a situation
when for first 64K flash segment compiler didn't set EINT explicitly. Thus
setting EIND at system startup makes sure we are safe.

Disabling interrupts at the very beginning of system init does not
make sense after reset since interrupts are disabled anyway. But
it is very convenient for debugging purposes, in situations when the
system misbehaves and ocassionally jumps to zero.

Sometimes I need UART to support binary data transfer, TCSETS/TCGETS allow
stwitching into binary mode.

Minor bug while working on Arduino Mega support. I didn't add support
for ARCH_HAVE_LEDS and have it undefined in .config. To my surprise compilation
failed since AVR code had a related function compiled in.

arch/avr/Makefile is adding extra EXEEXT to the
nuttix image file. This was not discovered since in most configurations
EXEEXT is empty string.