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/apalis-tk1_defconfig b/configs/apalis-tk1_defconfig
index a75a168..c1227a3 100644
--- a/configs/apalis-tk1_defconfig
+++ b/configs/apalis-tk1_defconfig
@@ -6,6 +6,7 @@
 CONFIG_FIT=y
 CONFIG_OF_SYSTEM_SETUP=y
 CONFIG_BOOTDELAY=1
+CONFIG_BOOTCOMMAND="run emmcboot; setenv fdtfile ${soc}-apalis-${fdt_board}.dtb && run distro_bootcmd"
 CONFIG_CONSOLE_MUX=y
 CONFIG_SYS_STDIO_DEREGISTER=y
 CONFIG_VERSION_VARIABLE=y