boot: add support for button commands

With the relatively new button API in U-Boot, it's now much easier to
model the common usecase of mapping arbitrary actions to different
buttons during boot - for example entering fastboot mode, setting some
additional kernel cmdline arguments, or booting with a custom recovery
ramdisk, to name a few.

Historically, this functionality has been implemented in board code,
making it fixed for a given U-Boot binary and requiring the code be
duplicated and modified for every board.

Implement a generic abstraction to run an arbitrary command during boot
when a specific button is pressed. The button -> command mapping is
configured via environment variables with the following format:

  button_cmd_N_name=<button label>
  button_cmd_N=<command to run>

Where N is the mapping number starting from 0. For example:

  button_cmd_0_name=vol_down
  button_cmd_0=fastboot usb 0

This will cause the device to enter fastboot mode if volume down is held
during boot.

After we enter the cli loop the button commands are no longer valid,
this allows the buttons to additionally be used for navigating a boot
menu.

Tested-by: Svyatoslav Ryhel <clamor95@gmail.com> # Tegra30
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>