cmd_extX: use common get_device_and_partition function

Convert ext2/4 load, ls, and write functions to use common device and
partition parsing function. With the common function "dev:part" can come
from the environment and a '-' can be used in that case.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>

include/ext_common.h

@@ -186,6 +186,8 @@ struct ext2_data {
 	struct ext2fs_node diropen;
 };
 
+extern unsigned long part_offset;
+
 int do_ext2ls(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[]);
 int do_ext2load(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[]);
 int do_ext4_load(cmd_tbl_t *cmdtp, int flag, int argc,