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>