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/kylin-rk3036_defconfig b/configs/kylin-rk3036_defconfig
index 100e73a..ab36b48 100644
--- a/configs/kylin-rk3036_defconfig
+++ b/configs/kylin-rk3036_defconfig
@@ -8,8 +8,11 @@
 CONFIG_SPL_STACK_R_ADDR=0x80000
 CONFIG_DEFAULT_DEVICE_TREE="rk3036-sdk"
 CONFIG_SPL_SYS_MALLOC_F_LEN=0x0
+# CONFIG_ANDROID_BOOT_IMAGE is not set
 # CONFIG_DISPLAY_CPUINFO is not set
 CONFIG_SPL_STACK_R=y
+CONFIG_FASTBOOT_FLASH=y
+CONFIG_FASTBOOT_FLASH_MMC_DEV=0
 # CONFIG_CMD_IMLS is not set
 CONFIG_CMD_GPT=y
 CONFIG_CMD_I2C=y