[bsp/rva8] make sure $BUILD_ROOT_DIR/output/target/vmm is a folder

bsp/realview-a8/mk.sh

@@ -6,6 +6,7 @@ KDIR=~/linux-git
 BUILD_ROOT_DIR=/temp-build/buildroot-2014.02/
 
 scons -j20
+mkdir -p $BUILD_ROOT_DIR/output/target/vmm
 cp rtthread.bin $BUILD_ROOT_DIR/output/target/vmm
 
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