libfdt: Add function to explicitly expand aliases

Kumar has already added alias expansion to fdt_path_offset().
However, in some circumstances it may be convenient for the user of
libfdt to explicitly get the string expansion of an alias.  This patch
adds a function to do this, fdt_get_alias(), and uses it to implement
fdt_path_offset().

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

tests/run_tests.sh

@@ -216,6 +216,7 @@ dtc_tests () {
 
     # Check aliases support in fdt_path_offset
     run_dtc_test -I dts -O dtb -o aliases.dtb aliases.dts
+    run_test get_alias aliases.dtb
     run_test path_offset_aliases aliases.dtb
 
     # Check /include/ directive