common: Move sorting functions to their own header file

These don't need to be in common.h so move them out into a new header.
Also add some missing comments.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/cmd/efi.c b/cmd/efi.c
index 919cb2f..ea239a0 100644
--- a/cmd/efi.c
+++ b/cmd/efi.c
@@ -9,6 +9,7 @@
 #include <efi.h>
 #include <errno.h>
 #include <malloc.h>
+#include <sort.h>
 
 static const char *const type_name[] = {
 	"reserved",