Kconfig: Migrate all of cmd/fastboot/Kconfig to defconfigs

- Move ANDROID_IMAGE_SUPPORT to top level Kconfig under images as it's
  not strictly part of fastboot.
- Add some defaults for the fastboot buffer location and size
- Migrate all options listed in cmd/fastboot/Kconfig
- Cleanup the README

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/xilinx_zynqmp_ep_defconfig b/configs/xilinx_zynqmp_ep_defconfig
index c43f032..c3ba5bf 100644
--- a/configs/xilinx_zynqmp_ep_defconfig
+++ b/configs/xilinx_zynqmp_ep_defconfig
@@ -17,6 +17,11 @@
 CONFIG_SPL_SYS_MALLOC_SIMPLE=y
 CONFIG_SPL_OS_BOOT=y
 CONFIG_SYS_PROMPT="ZynqMP> "
+CONFIG_FASTBOOT=y
+CONFIG_USB_FUNCTION_FASTBOOT=y
+CONFIG_CMD_FASTBOOT=y
+CONFIG_FASTBOOT_FLASH=y
+CONFIG_FASTBOOT_FLASH_MMC_DEV=0
 # CONFIG_CMD_CONSOLE is not set
 # CONFIG_CMD_IMLS is not set
 # CONFIG_CMD_XIMG is not set