uboot-imx

MA-16940 remove logically dead code

boot_buf was used to point to the memory allocated in do_boota function
to save boot partition data. now boot partition is loaded by AVB, this
boot_buf should be removed.

Change-Id: I614c412b5f7ed980d29106b364a8193c56db13dc
Signed-off-by: faqiang.zhu <faqiang.zhu@nxp.com>