[U-Boot] [PATCH V6 - Part 3 - 0/2] SPI flash enhancements: allow default bus and chip-selects

[Eric Nelson]

Patch 1 modifies the 'sf' command to allow a default bus and chip-select
to be specified by board headers. This allows a bare 'sf' probe command:
      U-Boot> sf probe
instead of the more cumbersome usage when a GPIO is tacked onto
the chip-select. Otherwise, this command-line would be needed
to specify GP3:19 on SabreLite:
      U-Boot> sf probe 0x5300

Patch 2 provides a description of usage and configuration of CONFIG_CMD_SF.

Signed-off-by: Eric Nelson <eric.nelson at boundarydevices.com>