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/bcm28155_ap_defconfig b/configs/bcm28155_ap_defconfig
index 10fa298..db1ad40 100644
--- a/configs/bcm28155_ap_defconfig
+++ b/configs/bcm28155_ap_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_ENABLE_ARM_SOC_BOOT0_HOOK=y
 CONFIG_TARGET_BCM28155_AP=y
+# CONFIG_ANDROID_BOOT_IMAGE is not set
 CONFIG_VERSION_VARIABLE=y
 # CONFIG_DISPLAY_CPUINFO is not set
 # CONFIG_DISPLAY_BOARDINFO is not set
@@ -10,7 +11,7 @@
 CONFIG_USB_FUNCTION_FASTBOOT=y
 CONFIG_CMD_FASTBOOT=y
 CONFIG_FASTBOOT_BUF_ADDR=0x80000000
-CONFIG_FASTBOOT_BUF_SIZE=0x7ff00000
+CONFIG_FASTBOOT_BUF_SIZE=0x7FF00000
 CONFIG_FASTBOOT_FLASH=y
 CONFIG_FASTBOOT_FLASH_MMC_DEV=0
 CONFIG_CMD_BOOTZ=y