gcc 10 compatibility for Drop alt (qmk#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.

tmk_core/protocol/arm_atsam/i2c_master.c

@@ -28,6 +28,9 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 #    define I2C_LED_USE_DMA 1  // Set 1 to use background DMA transfers for leds, Set 0 to use inline software transfers
 
+DmacDescriptor dmac_desc;
+DmacDescriptor dmac_desc_wb;
+
 static uint8_t i2c_led_q[I2C_Q_SIZE];  // I2C queue circular buffer
 static uint8_t i2c_led_q_s;            // Start of circular buffer
 static uint8_t i2c_led_q_e;            // End of circular buffer

tmk_core/protocol/arm_atsam/i2c_master.h

@@ -24,8 +24,8 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #    include "issi3733_driver.h"
 #    include "config.h"
 
-__attribute__((__aligned__(16))) DmacDescriptor dmac_desc;
-__attribute__((__aligned__(16))) DmacDescriptor dmac_desc_wb;
+extern __attribute__((__aligned__(16))) DmacDescriptor dmac_desc;
+extern __attribute__((__aligned__(16))) DmacDescriptor dmac_desc_wb;
 
 uint8_t I2C3733_Init_Control(void);
 uint8_t I2C3733_Init_Drivers(void);