Convert CONFIG_SUPPORT_EMMC_BOOT to Kconfig

This converts the following to Kconfig:
   CONFIG_SUPPORT_EMMC_BOOT

As requested by Michal Simek <michal.simek@xilinx.com>, these boards
have no eMMC so CONFIG_SUPPORT_EMMC_BOOT has not been migrated:

  xilinx_zynqmp_zc1275_revB
  xilinx_zynqmp_zc1751_xm018_dc4
  xilinx_zynqmp_zc1751_xm019_dc5
  xilinx_zynqmp_zcu100_revC
  xilinx_zynqmp_zcu102_rev1_0
  xilinx_zynqmp_zcu102_revA
  xilinx_zynqmp_zcu102_revB
  xilinx_zynqmp_zcu104_revA
  xilinx_zynqmp_zcu104_revC
  xilinx_zynqmp_zcu106_revA
  xilinx_zynqmp_zcu111_revA

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Acked-by: Lukasz Majewski <lukma@denx.de>
Acked-by: Patrick Delaunay <patrick.delaunay@st.com>
Acked-by: Ramon Fried <ramon.fried@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>