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 issues 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>
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