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/udoo_neo_defconfig b/configs/udoo_neo_defconfig
index 2f75361..4134586 100644
--- a/configs/udoo_neo_defconfig
+++ b/configs/udoo_neo_defconfig
@@ -10,6 +10,7 @@
 CONFIG_SPL_WATCHDOG_SUPPORT=y
 # CONFIG_CMD_BMODE is not set
 CONFIG_DISTRO_DEFAULTS=y
+CONFIG_BOOTCOMMAND="run findfdt; run distro_bootcmd"
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=arch/arm/mach-imx/spl_sd.cfg"
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_SPL=y