scripts: allow logging of commands with variables

In a lot of places, we need to call some commands with specific
variable settings, a-la:
var1=val1 var2=val2 /foo/bar/buz opt1 opt2

Unfortunately, we currently can not log the variable settings.

Enhance CT_DoExecLog with a crude heuristic that works pretty well
and that can also log setting variables.

Reported-by: ANDY KENNEDY <ANDY.KENNEDY@adtran.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 reporter_name="Alexander BIGGA"

 reporter_url="http://sourceware.org/ml/crossgcc/2008-07/msg00000.html"

 reporter_comment=""