package/llvm: set the path to llvm-config

package/llvm: set the path to llvm-config

While building llvm for the target (x86_64), the build failed due to
path poisoning (-I/usr/include/libxml2) while building NATIVE tools
(i.e for the host). The llvm package tries to build a tool for the host
with the cross-compiler which doesn't work when the paranoid toolchain
wrapper (BR2_COMPILER_PARANOID_UNSAFE_PATH) is enabled.

We know that llvm (target) needs llvm-tablegen and llvm-config built by
host-llvm, but only LLVM_TABLEGEN is provided by llvm.mk. Adding
LLVM_CONFIG_PATH=$(HOST_DIR)/bin/llvm-config for llvm (target)
fixes the path poisoining issue since llvm doesn't build the NATIVE
variant.

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Cc: Valentin Korenblit <valentinkorenblit@gmail.com>
Cc: Matthew Weber <matthew.weber@rockwellcollins.com>
Tested-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>