board: atmel: sama7g5ek: avoid rewriting of configured CONFIG_BOOTCOMMAND

Rewrite the CONFIG_BOOTCOMMAND only if it's not previously configured from
defconfig file.
This allows the user to select from defconfig/menuconfig the desired
boot command.
Adjust the current board defconfigs to reflect the default booting command
for the specific ENV configuration.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
3 files changed