build: teach envsetup.sh to respect a defined buildtools location

If BUILDTOOLS_PREBUILT_BIN is defined in the build.config, put it into
PATH as for the other prebuilt locations.

Bug: 135922132
Change-Id: Ib9df33b010301c1b652dc155eba78e448aed4e17
Signed-off-by: Matthias Maennich <maennich@google.com>
diff --git a/envsetup.sh b/envsetup.sh
index a9cca66..eb37f1e 100644
--- a/envsetup.sh
+++ b/envsetup.sh
@@ -44,6 +44,7 @@
 LZ4_PREBUILTS_BIN
 DTC_PREBUILTS_BIN
 LIBUFDT_PREBUILTS_BIN
+BUILDTOOLS_PREBUILT_BIN
 "
 
 for PREBUILT_BIN in ${PREBUILTS_PATHS}; do