Enable thumb build to reduce build size of u-boot.kwb.

Without this, u-boot.kwb overlaps where the u-boot environment is
stored, and updating the environment can break u-boot and vice versa.

  https://bugs.debian.org/897671
  https://lists.denx.de/pipermail/u-boot/2018-May/327497.html

Signed-off-by: Vagrant Cascadian <vagrant@debian.org>
Signed-off-by: Stefan Roese <sr@denx.de>
diff --git a/configs/sheevaplug_defconfig b/configs/sheevaplug_defconfig
index 608e05e..d77d198 100644
--- a/configs/sheevaplug_defconfig
+++ b/configs/sheevaplug_defconfig
@@ -36,3 +36,4 @@
 CONFIG_USB_EHCI_HCD=y
 CONFIG_USB_STORAGE=y
 CONFIG_LZMA=y
+CONFIG_SYS_THUMB_BUILD=y