Do not compile XT_NOOP if it is not needed.

Eliminates a warning message too.


git-svn-id: svn://svn.code.sf.net/p/amforth/code/trunk@1606 fd1c0b69-5d68-47f5-910c-86182ebb6ecd
Added a zero padding byte

git-svn-id: svn://svn.code.sf.net/p/amforth/code/trunk@1607 fd1c0b69-5d68-47f5-910c-86182ebb6ecd
More comments

git-svn-id: svn://svn.code.sf.net/p/amforth/code/trunk@1608 fd1c0b69-5d68-47f5-910c-86182ebb6ecd
Merge branch 'hq'

Conflicts:
	core/drivers/usart_common.asm