Account for non-existent 'build' dir in build_product

In some cases, especially when built as an RPM, the empty 'build'
directory may disappear, causing './build_product' to fail on

  ./build_product: line 395: cd: build: No such file or directory

Since we're already removing its content, just remove it completely
and re-create it from scratch. The '-f' of 'rm' causes it to
silently ignore non-existent 'build'.

Signed-off-by: Jiri Jaburek <comps@nomail.dom>

build_product

@@ -415,7 +415,8 @@ fi
 
 
 set -e
-rm -rf build/*
+rm -rf build
+mkdir build
 cd build
 cmake .. "${CMAKE_OPTIONS[@]}"
 $build_command "-j${jobs}" "${EXPLICIT_BUILD_TARGETS[@]}"