layerscape: ls1012ardb: only reserve ext4 fs as default firmware.bin

In Device/ls1012ardb IMAGES variable, there are two separate firmware
references to the same packages, while do mult-job compile, the same package
build process will arise conflict occasionally. So, only reserve one ext4 fs
as the default firmware.bin.

Signed-off-by: Yutang Jiang <yutang.jiang@nxp.com>