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/bcm23550_w1d_defconfig b/configs/bcm23550_w1d_defconfig
index 4895449..7091865 100644
--- a/configs/bcm23550_w1d_defconfig
+++ b/configs/bcm23550_w1d_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_ENABLE_ARM_SOC_BOOT0_HOOK=y
 CONFIG_TARGET_BCM23550_W1D=y
+# CONFIG_ANDROID_BOOT_IMAGE is not set
 CONFIG_VERSION_VARIABLE=y
 # CONFIG_DISPLAY_CPUINFO is not set
 # CONFIG_DISPLAY_BOARDINFO is not set
@@ -9,7 +10,7 @@
 CONFIG_USB_FUNCTION_FASTBOOT=y
 CONFIG_CMD_FASTBOOT=y
 CONFIG_FASTBOOT_BUF_ADDR=0x80000000
-CONFIG_FASTBOOT_BUF_SIZE=0x1d000000
+CONFIG_FASTBOOT_BUF_SIZE=0x1D000000
 CONFIG_FASTBOOT_FLASH=y
 CONFIG_FASTBOOT_FLASH_MMC_DEV=0
 CONFIG_CMD_BOOTZ=y