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_usbspl_defconfig b/configs/am335x_evm_usbspl_defconfig
index e3431db..c9041aa 100644
--- a/configs/am335x_evm_usbspl_defconfig
+++ b/configs/am335x_evm_usbspl_defconfig
@@ -4,6 +4,7 @@
 CONFIG_AM33XX=y
 # CONFIG_SPL_NAND_SUPPORT is not set
 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_ANDROID_BOOT_IMAGE is not set
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 CONFIG_VERSION_VARIABLE=y