Kconfig: Introduce USE_BOOTCOMMAND and migrate BOOTCOMMAND

We first introduce CONFIG_USE_BOOTCOMMAND, similar to
CONFIG_USE_BOOTARGS.  We then migrate CONFIG_BOOTCOMMAND for most
CONFIG_DISTRO_DEFAULT users.  In some cases platforms have a complex
scheme around this usage, and these have been defered for the moment so
that platform maintainers can work on a migration plan.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
diff --git a/configs/ls2080ardb_defconfig b/configs/ls2080ardb_defconfig
index 0fd3fee..36d2313 100644
--- a/configs/ls2080ardb_defconfig
+++ b/configs/ls2080ardb_defconfig
@@ -9,6 +9,7 @@
 CONFIG_BOOTDELAY=10
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="console=ttyS1,115200 root=/dev/ram0 earlycon=uart8250,mmio,0x21c0600 ramdisk_size=0x2000000 default_hugepagesz=2m hugepagesz=2m hugepages=256"
+# CONFIG_USE_BOOTCOMMAND is not set
 CONFIG_CMD_IMLS=y
 CONFIG_CMD_GREPENV=y
 CONFIG_CMD_EEPROM=y