Remove TF-M build folder before creating

Though on CI environments this is not needed because we start from a
clean workspace, on local machines may cause some trouble if build
build folder already exists, so remove the latter before creating it
again.

Change-Id: I5be60665067c927aebde662e7912d3ffe6261ef2
Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>
diff --git a/run-build.sh b/run-build.sh
index e931776..da571a7 100755
--- a/run-build.sh
+++ b/run-build.sh
@@ -48,6 +48,7 @@
 cd mbedtls
 git apply ../trusted-firmware-m/lib/ext/mbedcrypto/*.patch
 
+rm -rf ../trusted-firmware-m/build
 mkdir ../trusted-firmware-m/build
 cd ../trusted-firmware-m/build