include: download.mk: do not check PKG_MIRROR_HASH when set to "skip"

include: download.mk: do not check PKG_MIRROR_HASH when set to "skip"

In commit 042996b46bd41292ef1fa2d58e3b824a547f4c55 compilation of git repos is made to fail when PKG_MIRROR_HASH is not correct.

It looks like it was forgotten that in openwrt there is a posibility to set the PKG_MIRROR_HASH to "skip".
In this case the hash check should not be performed and compilation should continue as expected.

This is especially very usefull when doing local testing and development with git repos.

Signed-off-by: Matthias Franck <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20655
Signed-off-by: Robert Marko <[email protected]>