build: Add overload to clone repo and build in subdirectory

To build osmo-nitb we need to clone "openbsc" and then enter the
"openbsc" sub-directory. For building "mobile" and "virt_phy" of
OsmocomBB we will need to do something similar. Instead of added
more hardcoded paths, add a variant that allows to specify the
source directory.

Change-Id: I67fbd54296f2c68316bf8e89cfeb37d147193d53
diff --git a/contrib/jenkins-build-osmo-nitb.sh b/contrib/jenkins-build-osmo-nitb.sh
index 1e3ecc6..4f81fa8 100755
--- a/contrib/jenkins-build-osmo-nitb.sh
+++ b/contrib/jenkins-build-osmo-nitb.sh
@@ -10,6 +10,6 @@
 build_repo openggsn
 build_repo libsmpp34
 build_repo libosmo-sccp
-build_repo openbsc --enable-smpp --enable-osmo-bsc --enable-nat
+build_repo_dir openbsc openbsc --enable-smpp --enable-osmo-bsc --enable-nat
 
 create_bin_tgz "osmo-nitb osmo-bsc_mgcp"