Prevent redefine of HSE_VALUE if already user-overridden

[maxgerhardt]

Summary

Commercially sold Blackpill F411CE boards feature a 25MHz crystal, but if a board differs, HSE_VALUE can still be overridden while keeping everything else the same. With defensive programming like this, users of a custom (or differing commercially sold) board can easily use this variant with the same chip but just a slightly different crystal.

This PR fixes/implements the following bugs/features

• Feature: Let users redefine HSE_VALUE if needed

This topic e.g. appeared in https://community.platformio.org/t/env-blackpill-f401ce-consistently-creates-bad-binaries-that-hangs-the-mcu/18567 when a user was trying to use this Arduino core with a custom board with the 3 interchangable chips F401CC, F401CE and F411CE, all having a 25MHz crystal.

If a user would ever want to use a similiar setup but with a 25MHz crystal where the Arduino core uses the default 8MHz one or vice versa, it would be nice to always let the user redefine HSE_VALUE. With a simple #ifndef check, that's implemented here.

[fpistm]

Hi @maxgerhardt
in fact end user should use the generic variant if the board is a custom one.
The contributor who made those variant for the black pill define the correct clock value based on the hardware board definition.
Anyway, it is safe to allow this redefinition moreover I'm currently review/rewrite all the variant to be more generic and I already allow all redefiniton (just in case).