fastboot: Allow u-boot-style partitions

This adds support for partitions of the form "dev.hwpart:part" and
"dev#partname". This allows one to flash to eMMC boot partitions without
having to use CONFIG_FASTBOOT_MMC_BOOT1_SUPPORT. It also allows one to
flash to an entire device without needing CONFIG_FASTBOOT_MMC_USER_NAME.
Lastly, one can also flash MMC devices other than
CONFIG_FASTBOOT_FLASH_MMC_DEV.

Because devices can be specified explicitly, CONFIG_FASTBOOT_FLASH_MMC_DEV
is used only when necessary for existing functionality. For those cases,
fastboot_mmc_get_dev has been added as a helper function. This allows

There should be no conflicts with the existing system, but just in case, I
have ordered detection of these names after all existing names.

The fastboot_mmc_part test has been updated for these new names.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>