Add a "tidy" build option

Add a "tidy" build option

It is sometimes desireable to clean up the byproducts of the build
process without removing the executable results. "make clean" is
close, but leaves the build directory with a large number of
*.depend* files. This new build option invokes make clean, and then
removes the depend files.

Signed-off-by: Andy Fleming <afleming@freescale.com>