iMX: Fix compilation error when enabling SECURE_BOOT

Move the compilation of file fsl_validate.c in MACRO CONFIG_CMD_ESBC_VALIDATE.
This file should be compiled only when the above MACRO is defined

This caused a break in compilation of iMX platforms when compiling for SECURE_BOOT

Signed-off-by: Gaurav Rana <gaurav.rana@freescale.com>