kbuild: fix parallel build race caused by u-boot.cfg regeneration

Multiple people have reported intermittent build failure in parallel
building.

Kever Yang reported this issue some time ago [1], but I could not
get enough clue at that time.

This time, Richard Purdie provided a full build log [2], which was
very helpful for me to root-cause it.

The cause of the problem is commit 0d982c5853 ("Makefile: add
dependencies to regenerate u-boot.cfg when lost").

That commit added the 'cfg' as the prerequisite of the 'all' target,
so the parallel build tries to run it simultaneously, then regenerates
a symlink while building objects.

When u-boot.cfg is accidentally lost, let's rebuild it before
descending into any subdirectories.

Also, what is annoying is u-boot.cfg is currently regenerated every
time since it depends on FORCE. We can get rid of all the prerequisites
of u-boot.cfg because u-boot.cfg is rebuilt anyway as the byproduct of
auto.conf when a user updates the .config file.

[1] https://lists.denx.de/pipermail/u-boot/2018-June/330341.html
[2] https://autobuilder.yoctoproject.org/typhoon/#/builders/65/builds/160/steps/7/logs/step1b

Fixes: 0d982c5853 ("Makefile: add dependencies to regenerate u-boot.cfg when lost")
Reported-by: Kever Yang <kever.yang@rock-chips.com>
Reported-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

Makefile

@@ -534,7 +534,7 @@ 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
+u-boot.cfg spl/u-boot.cfg tpl/u-boot.cfg:
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.autoconf $(@)
 
 -include include/autoconf.mk
@@ -910,7 +910,7 @@ quiet_cmd_cfgcheck = CFGCHK  $2
 cmd_cfgcheck = $(srctree)/scripts/check-config.sh $2 \
 		$(srctree)/scripts/config_whitelist.txt $(srctree)
 
-all:		$(ALL-y) cfg
+all:		$(ALL-y)
 ifeq ($(CONFIG_DM_I2C_COMPAT)$(CONFIG_SANDBOX),y)
 	@echo >&2 "===================== WARNING ======================"
 	@echo >&2 "This board uses CONFIG_DM_I2C_COMPAT. Please remove"
@@ -1555,7 +1555,7 @@ ifneq ($(KBUILD_SRC),)
 endif
 
 # prepare2 creates a makefile if using a separate output directory
-prepare2: prepare3 outputmakefile
+prepare2: prepare3 outputmakefile cfg
 
 prepare1: prepare2 $(version_h) $(timestamp_h) \
                    include/config/auto.conf