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/nitrogen6dl_defconfig b/configs/nitrogen6dl_defconfig
index a2d8578..61e5ea4 100644
--- a/configs/nitrogen6dl_defconfig
+++ b/configs/nitrogen6dl_defconfig
@@ -9,6 +9,10 @@
 # CONFIG_SYS_STDIO_DEREGISTER is not set
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_HUSH_PARSER=y
+CONFIG_FASTBOOT=y
+CONFIG_USB_FUNCTION_FASTBOOT=y
+CONFIG_CMD_FASTBOOT=y
+CONFIG_FASTBOOT_BUF_ADDR=0x12000000
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set
 CONFIG_CMD_MEMTEST=y