ARM Fix pandaboard es and a4 revision ID

board_name environment variable was not getting set correctly for Pandaboard A4 and ES

Signed-off-by: David Batzle <dbatzle@dcbcyber.com>
CC: Albert Aribaud <albert.u.boot@aribaud.net>; Tom Rini <trini@ti.com>; Peter Robinson <pbrobinson@gmail.com>