binman: Respect pad-before property of section subentries

Other relevant properties (pad-after, offset, size, align, align-size,
align-end) already work since Pack() sets correct ranges for subentries'
data (.offset, .size variables), but some padding here is necessary to
align the data within this range to match the pad-before property.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>