package/urg: fix extraction commands

package/urg: fix extraction commands

Currently, the extraction commands entirely remove the urg directory,
which means the downloaded stamp will get removed, and thus a subsequent
build would try to re-download it.

It turns out that the directory extracted by urg is already correctly
named, so we just need to extract out of the build directory. This
highly simplifies the command.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>