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/CHIP_defconfig b/configs/CHIP_defconfig
index 253e2a8..83228bd 100644
--- a/configs/CHIP_defconfig
+++ b/configs/CHIP_defconfig
@@ -7,6 +7,7 @@
 CONFIG_DEFAULT_DEVICE_TREE="sun5i-r8-chip"
 CONFIG_SPL=y
 CONFIG_SPL_I2C_SUPPORT=y
+CONFIG_FASTBOOT_FLASH=y
 # CONFIG_CMD_IMLS is not set
 CONFIG_CMD_DFU=y
 CONFIG_CMD_USB_MASS_STORAGE=y