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/Kconfig b/Kconfig
index e57fad4..9b8a807 100644
--- a/Kconfig
+++ b/Kconfig
@@ -66,6 +66,7 @@
 	default y if ARCH_MESON
 	default y if ARCH_ROCKCHIP
 	default n
+	imply USE_BOOTCOMMAND
 	select CMD_BOOTZ if ARM && !ARM64
 	select CMD_BOOTI if ARM64
 	select CMD_DHCP