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/cgtqmx6eval_defconfig b/configs/cgtqmx6eval_defconfig
index 2dd5764..18f2ff8 100644
--- a/configs/cgtqmx6eval_defconfig
+++ b/configs/cgtqmx6eval_defconfig
@@ -23,6 +23,10 @@
 CONFIG_SPL_I2C_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="CGT-QMX6-Quad U-Boot > "
+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_DFU=y