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>
diff --git a/configs/sama7g5ek_mmc_defconfig b/configs/sama7g5ek_mmc_defconfig
index 83026eb..b5dd64c 100644
--- a/configs/sama7g5ek_mmc_defconfig
+++ b/configs/sama7g5ek_mmc_defconfig
@@ -19,6 +19,8 @@
 CONFIG_SD_BOOT=y
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="console=ttyS0,115200 root=/dev/mmcblk0p2 rw rootwait"
+CONFIG_USE_BOOTCOMMAND=y
+CONFIG_BOOTCOMMAND="fatload mmc 0:1 0x61000000 at91-sama7g5ek.dtb; fatload mmc 0:1 0x62000000 zImage; bootz 0x62000000 - 0x61000000"
 CONFIG_MISC_INIT_R=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y