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 0d982c58 ("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: 0d982c58 ("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>