Revert "include/shell.sh: remove getvar()"

It is more widely used than I thought.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43068 3c298f89-4303-0410-b956-a3cf2f4a3e73

include/shell.sh

+getvar() {
+	eval "echo \"\${$1}\""
+}
+
 var2file() {
 	local var
 	eval "var=\"\${$1}\""