configs: socfpga: soc64: Move CONFIG_BOOTCOMMAND to defconfig

CONFIG_BOOTCOMMAND have been moved to Kconfig.boot. This patch
move the CONFIG_BOOTCOMMAND macro from socfpga_soc64_common.h to
*_defconfig file for both Stratix 10 and Agilex.

Signed-off-by: Siew Chin Lim <elly.siew.chin.lim@intel.com>