bitbake: build/siggen.py: Avoid removing too many stamps when cleaning

bitbake: build/siggen.py: Avoid removing too many stamps when cleaning

The "*" part of the mask is to ensure we clean both any stamp, and any
setscene varient. It turns out we would also trample other tasks,
e.g. do_package_write could trample do_package_write_rpm. do_package also
tramples do_package_write_* but this is less of an issue since the other
tasks depend on it.

Rather than use the wildcard, we can just use a list instead.

[YOCTO #3484]

(Bitbake rev: c14d831ea3f625e9a47266a0c4e6deefc924ca5a)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>