botan: fix install directory

Using $(STAGING_DIR)/usr and $(TARGET_DIR)/usr as the DESTDIR value
causes Botan to be installed in $(STAGING_DIR)/usr/usr and
$(TARGET_DIR)/usr/usr, which obviously isn't correct. Let's fix that
by passing the appropriate DESTDIR values.

Signed-off-by: DUPONCHEEL Sébastien <sebastien.duponcheel@corp.ovh.com>
[Thomas: extend commit log.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>