build: simplify the setup / update of repos

The `update' target has made several advances in the last months,
so that it can be used to collect new changes, switch to the correct
branch and also clone repositories as needed.

README.md

@@ -15,13 +15,9 @@ tasks require a root user.  Do the following to grab the repositories
 
     # pkg install git
     # cd /usr
-    # rm -rf src ports
-    # git clone https://github.com/opnsense/plugins
-    # git clone https://github.com/opnsense/ports
     # git clone https://github.com/opnsense/tools
-    # git clone https://github.com/opnsense/core
-    # git clone https://github.com/opnsense/src
     # cd tools
+    # make update
 
 TL;DR
 =====
@@ -171,9 +167,9 @@ distribution files before running the actual build:
 
     # make distfiles
 
-Updating the individual repositories can be done as follows:
+Updating all or individual repositories can be done as follows:
 
-    # make update-<repo1>[,...]
+    # make update[-<repo1>[,...]]
 
 Available update options are: core, plugins, ports, src, tools
 

build/common.sh

@@ -245,10 +245,14 @@ git_fetch()
 
 git_clone()
 {
-	if [ -d "${1}" ]; then
+	if [ -d "${1}/.git" ]; then
 		return
 	fi
 
+	if [ -d "${1}" ]; then
+		rm -r "${1}"
+	fi
+
 	echo ">>> Cloning ${1}:"
 
 	git clone "${PRODUCT_GITBASE}/$(basename ${1})" ${1}