base-files: remove some bashisms

"[[" is a bash extension for test. As the ash-implementation is
not fully compatible we drop its usage.
Also change to "=" for simple test, which is sufficient. (see d6ac8ca7)

Signed-off-by: Sven Roederer <devel-sven@geroedel.de>
[split patch, removed shebang]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

package/base-files/files/lib/functions/caldata.sh

@@ -69,7 +69,7 @@ caldata_valid() {
 	local expected="$1"
 
 	magic=$(hexdump -v -n 2 -e '1/1 "%02x"' /lib/firmware/$FIRMWARE)
-	[[ "$magic" == "$expected" ]]
+	[ "$magic" = "$expected" ]
 	return $?
 }