Make build.sh friendlier

- Use version variables for dependencies.
- Align with other projects.

build.sh

@@ -1,9 +1,24 @@
 #! /bin/sh
 
-LIBDIR=../tkey-libs
+tkey_libs_version="v0.0.1"
+signer_version="v0.0.7"
 
-git clone -b v0.0.1 https://github.com/tillitis/tkey-libs.git ../tkey-libs
-git clone -b v0.0.7 https://github.com/tillitis/tkey-device-signer.git ../tkey-device-signer
+printf "Building tkey-libs with version: %s\n" "$tkey_libs_version"
+printf "Building signer with version: %s\n" "$signer_version"
+
+if [ -d ../tkey-libs ]
+then
+    (cd ../tkey-libs; git checkout main; git pull; git checkout "$tkey_libs_version")
+else
+    git clone -b "$tkey_libs_version" https://github.com/tillitis/tkey-libs.git ../tkey-libs
+fi
+
+if [ -d ../tkey-device-signer ]
+then
+    (cd ../tkey-device-signer; git checkout main; git pull; git checkout "$signer_version")
+else
+    git clone -b "$signer_version" https://github.com/tillitis/tkey-device-signer.git ../tkey-device-signer
+fi
 
 make -j -C ../tkey-libs
 make -j -C ../tkey-device-signer