configs: ast2600: Boot kernel FIT in DRAM

AST2600 leverages the FIT hash/signature verification to fulfill
secure boot trust chain. To improve the performance and save SW
code size for those crypto operations, the two HW crypto engine,
HACE and ACRY, are enabled.

However, both of the engines can only access to data stored in
DRAM space. Therefore, we need to move the FIT image into DRAM
before the booting.

This patch update the CONFIG_BOOTCOMMAND to execute the pre-defined
ENV variable which consists of FIT image copy to memory and booting.

Signed-off-by: Chia-Wei Wang <chiawei_wang@aspeedtech.com>