cmd: jffs2: Rename command ls to fsls

Rename command ls to fsls as its conflicting with
generic file systesm command ls and this is causing
compilation failure as below, if both are enabled
and this patch fixes it.

cmd/jffs2.o:(.u_boot_list_2_cmd_2_ls+0x0):
multiple definition of `_u_boot_list_2_cmd_2_ls'
cmd/fs.o:(.u_boot_list_2_cmd_2_ls+0x0):first defined here
scripts/Makefile.build:359: recipe for target 'cmd/built-in.o'
failed
make[1]: *** [cmd/built-in.o] Error 1

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>