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/am335x_evm_spiboot_defconfig b/configs/am335x_evm_spiboot_defconfig
index ad79d6c..d1dfa89 100644
--- a/configs/am335x_evm_spiboot_defconfig
+++ b/configs/am335x_evm_spiboot_defconfig
@@ -6,6 +6,7 @@
 CONFIG_SPL_SPI_FLASH_SUPPORT=y
 CONFIG_SPL_SPI_SUPPORT=y
 CONFIG_DISTRO_DEFAULTS=y
+CONFIG_BOOTCOMMAND="if test ${boot_fit} -eq 1; then run update_to_fit; fi; run findfdt; run init_console; run envboot; run distro_bootcmd"
 CONFIG_SYS_EXTRA_OPTIONS="SPI_BOOT"
 CONFIG_SPI_BOOT=y
 CONFIG_SYS_CONSOLE_INFO_QUIET=y