scripts/checkpatch: Limit which feature test macros we prevent

Unfortunately this check as it is today is causing trouble, while
not checking too well for what it intended. Let's reduce its scope
until a better solution has been found.

Background:
This check intends to ensure coding guidelines Rules A.4 and A.5
are followed, but how it is implemented it does not work well enough.
1. These rules only apply to the kernel and some other parts of the
embedded codebase respectively, but this check is performed on the
whole tree.
2. This check works under the assumption that any attempt to set
these macros in source files is a violation of these rules, while
this is not necessary the case, as there are legitimate uses for these.
(Specially for _POSIX_C_SOURCE and _XOPEN_SOURCE)

This check also fails to detect these macros being set in cmake files,
so if users are faced with this failure they can trivially bypass it.

Having a CI check which produces too many false positives, while
at the same time being very easy to bypass is not a desirable situation
as that can result in lack of trust for this type of checks,
and an overall tendency to override these CI faults,
and overlooking actual violations of these rules by reviewers.

This check was originally added in
b021dece

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>