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>
diff --git a/drivers/mmc/Kconfig b/drivers/mmc/Kconfig
index c34dd5d..e96e693 100644
--- a/drivers/mmc/Kconfig
+++ b/drivers/mmc/Kconfig
@@ -78,6 +78,12 @@
 	  Enable support for reading, writing and programming the
 	  key for the Replay Protection Memory Block partition in eMMC.
 
+config SUPPORT_EMMC_BOOT
+	bool "Support some additional features of the eMMC boot partitions"
+	help
+	  Enable support for eMMC boot partitions. This also enables
+	  extensions within the mmc command.
+
 config MMC_IO_VOLTAGE
 	bool "Support IO voltage configuration"
 	help