config: introduce a generic $bootcmd

This generic $bootcmd, and associated support macros, automatically
searches a defined set of storage devices (or network protocols) for an
extlinux configuration file or U-Boot boot script in various standardized
locations. Distros that install such a boot config file/script in those
standard locations will get easy-to-set-up booting on HW that enables
this generic $bootcmd.

Boards can define the set of devices from which boot is attempted, and
the order in which they are attempted. Users may later customize this
set/order by edting $boot_targets.

Users may interrupt the boot process and boot from a specific device
simply by executing e.g.:

$ run bootcmd_mmc1
or:
$ run bootcmd_pxe

This patch was originally written by Dennis Gilmore based on Tegra and
rpi_b boot scripts. I have made the following modifications since then:

* Boards must define the BOOT_TARGET_DEVICES macro in order to specify
  the set of devices (and order) from which to attempt boot. If needed,
  we can define a default directly in config_distro_bootcmd.h.

* Removed $env_import and related variables; nothing used them, and I
  think it's better for boards to pre-load an environment customization
  file using CONFIG_PREBOOT if they need.

* Renamed a bunch of variables to suit my whims:-)

Signed-off-by: Dennis Gilmore <dennis@ausil.us>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Acked-by: Simon Glass <sjg@chromium.org>