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/igep0032_defconfig b/configs/igep0032_defconfig
index 0521039..c295dea 100644
--- a/configs/igep0032_defconfig
+++ b/configs/igep0032_defconfig
@@ -2,6 +2,7 @@
 CONFIG_ARCH_OMAP2PLUS=y
 CONFIG_TARGET_OMAP3_IGEP00X0=y
 CONFIG_DISTRO_DEFAULTS=y
+CONFIG_BOOTCOMMAND="run findfdt; run distro_bootcmd"
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_BOOTDELAY=3
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y