Certain projects may want to prevent the usage of certain modules from
being used. Here are two examples of when this may occur:
1. The Project has it's own fdt library and does not want developer's
using the version of libfdt included with LK
2. The project does not want developers using mincrypt.
`DENY_MODULES` is a list which developers can set in their own project
makefiles which is checked on each module inclusion. If that module is
on the deny list, it causes a build failure.
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