util: add class for storing extensions strings
This utility will be used in future commits.
Also add -Wno-undefined to the linker flags to prevent leaving undefined symbols in the layer .so file. Fix one issue highlighted while using this flag.
Change-Id: I682e266f7b3f313742cb2da83e5ad2569fc72da3 Signed-off-by: Joe Jenner-Bailey joe.jenner-bailey@arm.com Signed-off-by: Matteo Franchin matteo.franchin@arm.com Signed-off-by: Rosen Zhelev rosen.zhelev@arm.com