boards: doc: Clarify a generic statement

This sentence ("Other hardware features are not supported by the
Zephyr kernel."), which could be found in a high number of boards
documentation, is misleading on two levels:
- peripheral support is not a kernel business
- in most of cases, features are actually supported but not enabled.

Fix this.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>