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/vexpress_ca5x2_defconfig b/configs/vexpress_ca5x2_defconfig
index 3f8bc9c..68d08a0 100644
--- a/configs/vexpress_ca5x2_defconfig
+++ b/configs/vexpress_ca5x2_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_TARGET_VEXPRESS_CA5X2=y
 CONFIG_DISTRO_DEFAULTS=y
+CONFIG_BOOTCOMMAND="run distro_bootcmd; run bootflash"
 # CONFIG_DISPLAY_CPUINFO is not set
 # CONFIG_DISPLAY_BOARDINFO is not set
 # CONFIG_CMD_CONSOLE is not set