Fix missing leading whitespace with ':append'

Mitigate occurences where ':append' operator is used and leading
whitespace character is obviously missing, risking inadvertent
string concatenation.

(From OE-Core rev: fcd340ec)

Signed-off-by: Niko Mauno <niko.mauno@vaisala.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>