device-fsl

MA-15150 Clean output before build u-boot images

Clean the output files before build new u-boot targets.

Test: build with imx8mm.

Change-Id: If3dfc9bc55c29086e95e7c07a154e851261fa6c9
Signed-off-by: Ji Luo <ji.luo@nxp.com>
Reviewed-on: http://androidsource.ap.freescale.net/project/9287
Reviewed-by: guoyin.chen <guoyin.chen@nxp.com>