Log Message:
Add WAF_REPLACE_EXECUTABLE in waf.mk

The waf tool has been designed as a bundled 3rd party script in a software
tarball. For this reason some vendors freely modify these scripts and are
technically incompatible with the stock one.

Allow to use waf.mk goodies in such circumstances by specifying
"WAF_REPLACE_EXECUTABL=no" in a package's Makefile. Retain the default
behavior of replacing or copying waf into a package unchanged.

Reviewed by <wiz>