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/sandbox_spl_defconfig b/configs/sandbox_spl_defconfig
index 468d556..be4a85d 100644
--- a/configs/sandbox_spl_defconfig
+++ b/configs/sandbox_spl_defconfig
@@ -6,6 +6,7 @@
 CONFIG_SANDBOX_SPL=y
 CONFIG_DEFAULT_DEVICE_TREE="sandbox"
 CONFIG_DISTRO_DEFAULTS=y
+CONFIG_ANDROID_BOOT_IMAGE=y
 CONFIG_FIT=y
 CONFIG_FIT_SIGNATURE=y
 CONFIG_FIT_VERBOSE=y