autoboot: add CONFIG_AUTOBOOT to allow to not compile autoboot.c

Since commit bb597c0e ("common: bootdelay: move CONFIG_BOOTDELAY
into a Kconfig option"), CONFIG_BOOTDELAY is defined for all boards.

Prior to that commit, it was allowed to unset CONFIG_BOOTDELAY to
not compile common/autoboot.c, as described in common/Makefile:

  # This option is not just y/n - it can have a numeric value
  ifdef CONFIG_BOOTDELAY
  obj-y += autoboot.o
  endif

It was a bit odd to enable/disable code with an integer type option,
but it was how this option worked before that commit, and several
boards actually unset it to opt out of the autoboot feature.

This commit adds a new bool option, CONFIG_AUTOBOOT, and makes
CONFIG_BOOTDELAY depend on it.

I chose "default y" for this option because most boards use the
autoboot.  I added "# CONFIG_AUTOBOOT is not set" for the boards that
had not set CONFIG_BOOTDELAY prior to the bad commit.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>