rm_work: handle non-existant stamps directory

rm_work: handle non-existant stamps directory

Since 'build/siggen: Rework stamps functions'[1] the stamps directory
for each recipe is only created if there are stamps to be written. Some
recipes - such as package-index - only have nostamp tasks so the stamps
directory is never created, resulting in the rm_work task trying to cd
into a directory that doesn't exist.

Resolve this by simply checking the stamps directory exists before
trying to delete files in it.

[1] bitbake c79ecec580e4c2a141ae483ec0f6448f70593dcf

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>