cmake: Fix issues when IDF_PATH is not set in environment

Support cases where IDF_PATH may be passed in on the cmake command line,
or inferred from a (hardcoded absolute or relative) path to project.cmake

components/bootloader/project_include.cmake

@@ -17,7 +17,7 @@ externalproject_add(bootloader
     # TODO: support overriding the bootloader in COMPONENT_PATHS
     SOURCE_DIR "${IDF_PATH}/components/bootloader/subproject"
     BINARY_DIR "${bootloader_build_dir}"
-    CMAKE_ARGS -DSDKCONFIG=${SDKCONFIG} -DIDF_PATH="${IDF_PATH}"
+    CMAKE_ARGS -DSDKCONFIG=${SDKCONFIG} -DIDF_PATH=${IDF_PATH}
     INSTALL_COMMAND ""
     BUILD_ALWAYS 1  # no easy way around this...
     BUILD_BYPRODUCTS ${bootloader_binary_files}