Makefile: add dependencies to regenerate u-boot.cfg when lost

When running a 'make clean' or carelessly removing u-boot.cfg, all
future make invocations (until autoconf is regenerated) will print
an error for a missing u-boot.cfg due to missing rules and dependencies.

This commit adds (i) an explicit rule dependency from all (which will
invokes the configuration checker) to cfg, and (b) adds a rule to
invoke scripts/Makefile.autoconf to regenerate u-boot.cfg.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Bin Meng <bmeng.cn@gmail.com>

Makefile

@@ -516,6 +516,9 @@ include/config/%.conf: $(KCONFIG_CONFIG) include/config/auto.conf.cmd
 	@# Otherwise, 'make silentoldconfig' would be invoked twice.
 	$(Q)touch include/config/auto.conf
 
+u-boot.cfg spl/u-boot.cfg tpl/u-boot.cfg: include/config.h FORCE
+	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.autoconf $(@)
+
 -include include/autoconf.mk
 -include include/autoconf.mk.dep
 
@@ -851,7 +854,7 @@ quiet_cmd_cfgcheck = CFGCHK  $2
 cmd_cfgcheck = $(srctree)/scripts/check-config.sh $2 \
 		$(srctree)/scripts/config_whitelist.txt $(srctree)
 
-all:		$(ALL-y)
+all:		$(ALL-y) cfg
 ifeq ($(CONFIG_DM_I2C_COMPAT)$(CONFIG_SANDBOX),y)
 	@echo "===================== WARNING ======================"
 	@echo "This board uses CONFIG_DM_I2C_COMPAT. Please remove"