prd-flash: eliminate superfluous whitespace
prd-flash: eliminate superfluous whitespace

According to bitbake user-manual (1), "+=" inserts a whitespace between
previous content and appended string, so avoid starting string to append
with a new whitespace.

(1): https://www.yoctoproject.org/docs/latest/bitbake-user-manual/bitbake-user-manual.html#appending-and-prepending

Signed-off-by: Jens Rehsack <69d49c504a9219a55b8be0f905e8fe3c32ffb57f@netbsd.org>

rehsack/meta-jens,rehsack/meta-jens,rehsack/meta-jens