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/fennec-rk3288_defconfig b/configs/fennec-rk3288_defconfig
index a52f1f5..e00b62d 100644
--- a/configs/fennec-rk3288_defconfig
+++ b/configs/fennec-rk3288_defconfig
@@ -7,11 +7,14 @@
 CONFIG_SPL_STACK_R_ADDR=0x80000
 CONFIG_DEFAULT_DEVICE_TREE="rk3288-fennec"
 CONFIG_DEBUG_UART=y
+# CONFIG_ANDROID_BOOT_IMAGE is not set
 CONFIG_SILENT_CONSOLE=y
 CONFIG_CONSOLE_MUX=y
 # CONFIG_DISPLAY_CPUINFO is not set
 CONFIG_SPL_STACK_R=y
 CONFIG_SPL_STACK_R_MALLOC_SIMPLE_LEN=0x2000
+CONFIG_FASTBOOT_FLASH=y
+CONFIG_FASTBOOT_FLASH_MMC_DEV=0
 # CONFIG_CMD_IMLS is not set
 CONFIG_CMD_GPIO=y
 CONFIG_CMD_GPT=y