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/ti816x_evm_defconfig b/configs/ti816x_evm_defconfig
index be9c21f..b9bb57f 100644
--- a/configs/ti816x_evm_defconfig
+++ b/configs/ti816x_evm_defconfig
@@ -15,6 +15,7 @@
 CONFIG_BOOTDELAY=3
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="console=ttyO2,115200n8 noinitrd earlyprintk"
+# CONFIG_USE_BOOTCOMMAND is not set
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 CONFIG_VERSION_VARIABLE=y
 # CONFIG_DISPLAY_BOARDINFO is not set