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/evb-rk3229_defconfig b/configs/evb-rk3229_defconfig
index 0e9cea7..5a658a1 100644
--- a/configs/evb-rk3229_defconfig
+++ b/configs/evb-rk3229_defconfig
@@ -12,7 +12,8 @@
 # CONFIG_DISPLAY_CPUINFO is not set
 CONFIG_SPL_STACK_R=y
 CONFIG_SPL_STACK_R_MALLOC_SIMPLE_LEN=0x200
-CONFIG_FASTBOOT=y
+CONFIG_FASTBOOT_FLASH=y
+CONFIG_FASTBOOT_FLASH_MMC_DEV=0
 # CONFIG_CMD_IMLS is not set
 CONFIG_CMD_GPT=y
 CONFIG_CMD_MMC=y