device-fsl

MA-15613 avoid uboot-imx*-dual.imx being generated

With dual bootloader target, uboot-imx*-dual.imx will be generated, and
it's not used by us since it does not have neccesary firmware such as
ATF. Take 8mm_evk as an example, "u-boot-imx8mm-dual.imx" is generated
and copied to daily image directory.

change the log of uboot.mk to avoid the generation of these images.

Test on : evk_7ulp, evk_8mm, evk_8mn, mek_8q_car2

Change-Id: Iea247467767f36b4543d62c968c19dc045a79eef
Signed-off-by: faqiang.zhu <faqiang.zhu@nxp.com>
Reviewed-on: http://androidsource.ap.freescale.net/project/10233
Reviewed-by: ji.luo <ji.luo@nxp.com>