Impossible to install to a buildroot

See intel/confidential-computing.sgx#383 for the lack of a `make install` target. In working around that, I created the installer using `make sdk_install_pkg` then installed that.

I installed the SDK to a temporary directory so I could package it with RPM. However, the following files had the temporary root hardcoded.

```
/usr/bin/sgx-gdb:GDB_SGX_PLUGIN_PATH=/sgxsdk/lib64/gdb-sgx-plugin
/usr/bin/sgx-gdb:SGX_LIBRARY_PATH=/sgxsdk/lib64
/usr/pkgconfig/libsgx_uae_service_sim.pc:prefix=/sgxsdk
/usr/pkgconfig/libsgx_urts_sim.pc:prefix=/sgxsdk
/usr/pkgconfig/libsgx_uae_service.pc:prefix=/sgxsdk
/usr/pkgconfig/libsgx_urts.pc:prefix=/sgxsdk
/usr/uninstall.sh:rm -fr /sgxsdk 2> /dev/null
/usr/environment:export SGX_SDK=/sgxsdk
```

Please provide a way to make a buildroot installation. Preferrably, fix intel/confidential-computing.sgx#383.