spl: mmc: make eMMC HW boot partition configuration optional

spl: mmc: make eMMC HW boot partition configuration optional

Loading U-Boot on i.MX8QXP MEK board is broken since recent changes
in spl_mmc:

  U-Boot SPL 2019.10-00162-gff5bd397e4 (Oct 18 2019 - 15:50:45 +0200)
  Normal Boot
  WDT:   Not found!
  Trying to boot from MMC2_2
  Load image from MMC/SD 0x46400
  spl: mmc partition switch failed
  SPL: failed to boot from all boot devices
  ### ERROR ### Please RESET the board ###

The newly added CONFIG_SYS_MMCSD_RAW_MODE_EMMC_BOOT_PARTITION option
is selected automatically and overrides the EXT_CSC_PART_CONFIG
configurations with default value, always selecting the eMMC boot
partition 1. Boards which place U-Boot image in other partitions
became not bootable.

Fix this by making the eMMC HW boot partition selection optional.

Fixes: 17241ea0543a (spl: mmc: Add option to set eMMC HW boot partition)
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Signed-off-by: Anatolij Gustschin <agust@denx.de>