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/novena_defconfig b/configs/novena_defconfig
index b6a03dc..3084fef 100644
--- a/configs/novena_defconfig
+++ b/configs/novena_defconfig
@@ -12,6 +12,7 @@
 CONFIG_SPL_FAT_SUPPORT=y
 CONFIG_CMD_HDMIDETECT=y
 CONFIG_DISTRO_DEFAULTS=y
+CONFIG_BOOTCOMMAND="run distro_bootcmd ; run net_nfs"
 CONFIG_FIT=y
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/mach-imx/spl_sd.cfg,MX6Q"
 CONFIG_USE_BOOTARGS=y