gcc 10 compatibility for Drop alt (#9485)

* Split dmac_desc declaration and definition

According to the official documentation[1] gcc 10 is more strict about
correct extern usage.

I've had to move the definition of dmac_desc and dmac_desc_wb from
i2c_master.h to the corresponding .c file. This could be an issue if
anyone includes the i2c_master.h file without liking with the object
file.

[1]: https://gcc.gnu.org/gcc-10/porting_to.html

* Remove the keymap_config definition from keymaps

The keymap_config def was conflicting with the one found in
tmk_core/common/magic.c. Declaring it extern in magic.c breaks a bunch
of keyboard that rely on that declaration (like the ergodox). Instead
I've removed the one found in the keymap.c of the massdrop alt.

The same change will have to be made to other keyboards.

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