sandbox: Force command sections to be 4-byte aligned

By default sections are 16-byte aligned on some architectures, but the
command name structure (struct cmd_tbl_s) does not have padding to
16 bytes. This reduces the alignment to 4-bytes so that the command
table can be accessed correctly on any architecture.

(Note: this needs doing properly)

Signed-off-by: Simon Glass <sjg@chromium.org>