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/sniper_defconfig b/configs/sniper_defconfig
index 63c9f7f..f24153b 100644
--- a/configs/sniper_defconfig
+++ b/configs/sniper_defconfig
@@ -12,6 +12,13 @@
 # CONFIG_SPL_EXT_SUPPORT is not set
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="sniper # "
+CONFIG_FASTBOOT=y
+CONFIG_USB_FUNCTION_FASTBOOT=y
+CONFIG_CMD_FASTBOOT=y
+CONFIG_FASTBOOT_BUF_ADDR=0x82000000
+CONFIG_FASTBOOT_BUF_SIZE=0x2000000
+CONFIG_FASTBOOT_FLASH=y
+CONFIG_FASTBOOT_FLASH_MMC_DEV=0
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set
 # CONFIG_CMD_FLASH is not set