Add simple hwconfig infrastructure

This patch implements simple hwconfig infrastructure: an
interface for software knobs to control a hardware.

This is very simple implementation, i.e. it is implemented
via `hwconfig' environment variable. Later we could write
some "hwconfig <enable|disable|list>" commands, ncurses
interface for Award BIOS-like interface, and frame-buffer
interface for AMI GUI[1] BIOS-like interface with mouse
support[2].

Current implementation details/limitations:

1. Doesn't support options dependencies and mutual exclusion.
   We can implement this by integrating apt-get[3] into the
   u-boot. But I didn't bother yet.

2. Since we don't implement hwconfig command, i.e. we're working
   with the environement directly, there is no way to tell that
   toggling a particular option will need a reboot to take
   an effect. So, for now it's advised to always reboot the
   target after modifying hwconfig variable.

3. We support hwconfig options with arguments. For example,

   set hwconfig dr_usb:mode=peripheral,phy_type=ulpi

   That means:
   - dr_usb - enable Dual-Role USB controller;
   - dr_usb:mode=peripheral - USB in Function mode;
   - dr_usb:phy_type=ulpi - USB should work with ULPI PHYs;

The purpose of this simple implementation is to define some
internal API and then we can continue improving user experience
by adding more mature interface, like hwconfig command with
bells and whistles. Or not adding, if we feel that current
interface fits its needs.

[1] http://en.wikipedia.org/wiki/American_Megatrends


[2] Regarding ncurses and GUI with mouse support -- I'm just
    kidding.
[3] The comment regarding apt-get is also a joke, meaning that
    dependency tracking could be non-trivial. For example, for
    enabling HW feature X we may need to disable Y, and turn Z
    into reduced mode (like RMII-only interface for ethernet,
    no MII).

    It's quite trivial to implement simple cases though.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Acked-by: Kim Phillips <kim.phillips@freescale.com>