make: Add project number to Doxygen Documentation

The variable BUILD_STRING in an input to the Doxyfile that is used to generate

the documentation but it was not being exported by the Makefile so it was not being used/detected

Signed-off-by: Tomás Agustín González Orlando <tomasagustin.gonzalezorlando@arm.com>
Change-Id: I22733aba018d53479710efa069f009032e4b1c36