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/birdland_bav335a_defconfig b/configs/birdland_bav335a_defconfig
index 2e6c8a6..44f4eac 100644
--- a/configs/birdland_bav335a_defconfig
+++ b/configs/birdland_bav335a_defconfig
@@ -23,6 +23,10 @@
 CONFIG_SPL_POWER_SUPPORT=y
 CONFIG_SPL_YMODEM_SUPPORT=y
 CONFIG_HUSH_PARSER=y
+CONFIG_FASTBOOT=y
+CONFIG_USB_FUNCTION_FASTBOOT=y
+CONFIG_CMD_FASTBOOT=y
+CONFIG_FASTBOOT_BUF_ADDR=0x82000000
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set
 CONFIG_CMD_SPL=y