This was already used by some SPL targets, and allows the pad amount to
be specified by board config headers rather than only in makefile
fragments.

Also supply a pad-to of zero if the variable is undefined.  It works
without this, but this avoids relying on undocumented behavior.
Signed-off-by: NScott Wood <scottwood@freescale.com>
Signed-off-by: NKim Phillips <kim.phillips@freescale.com>