libfdt: Fix handling of trailing / in fdt_path_offset()

Currently, fdt_path_offset() returns FDL_ERR_BADOFFSET if given a path
with a trailing '/'.  In particular this means that
fdt_path_offset("/") returns FDT_ERR_BADOFFSET rather than 0 as one
would expect.

This patch fixes the function to accept and ignore trailing '/'
characters.  As well as allowing fdt_path_offset("/") this means that
fdt_path_offset("/foo/") will return the same as
fdt_path_offset("/foo") which seems in keeping with the principle of
least surprise.

This also adds a testcase to ensure that fdt_path_offset("/") returns
0 as it should.

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

libfdt/fdt_ro.c

@@ -154,7 +154,7 @@ int fdt_path_offset(const void *fdt, const char *path)
 		while (*p == '/')
 			p++;
 		if (! *p)
-			return -FDT_ERR_BADPATH;
+			return offset;
 		q = strchr(p, '/');
 		if (! q)
 			q = end;