Add bootscript support to esbc_validate.

1. Default environment will be used for secure boot flow
 which can't be edited or saved.
2. Command for secure boot is predefined in the default
 environment which will run on autoboot (and autoboot is
 the only option allowed in case of secure boot) and it
 looks like this:
 #define CONFIG_SECBOOT \
 "setenv bs_hdraddr 0xe8e00000;"                 \
 "esbc_validate $bs_hdraddr;"                    \
 "source $img_addr;"                             \
 "esbc_halt;"
 #endif
3. Boot Script can contain esbc_validate commands and bootm command.
 Uboot source command used in default secure boot command will
 run the bootscript.
4. Command esbc_halt added to ensure either bootm executes
 after validation of images or core should just spin.

Signed-off-by: Ruchika Gupta <ruchika.gupta@freescale.com>
Signed-off-by: Gaurav Rana <gaurav.rana@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>