Add SPDX license identifiers

Add license information to files. Mostly automated with:

    find src/ -name '*.[c,h]' | xargs sed -i '1s,^,// SPDX-License-Identifier: GPL-3.0-only\n\n,'
    find src/ -name '*.mk' | xargs sed -i '1s,^,# SPDX-License-Identifier: GPL-3.0-only\n\n,'

src/common/i2c.c

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
 #include <common/i2c.h>
 
 int i2c_recv(struct I2C * i2c, uint8_t addr, uint8_t* data, int length) {

src/common/include/common/command.h

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
 #ifndef _COMMON_COMMAND_H
 #define _COMMON_COMMAND_H
 

src/common/include/common/debug.h

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
 #ifndef _COMMON_DEBUG_H
 #define _COMMON_DEBUG_H
 

src/common/include/common/i2c.h

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
 #ifndef _COMMON_I2C_H
 #define _COMMON_I2C_H
 

src/common/include/common/keymap.h

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
 #ifndef _COMMON_KEYMAP_H
 #define _COMMON_KEYMAP_H
 

src/common/include/common/macro.h

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
 #ifndef _COMMON_MACRO_H
 #define _COMMON_MACRO_H
 

src/common/include/common/version.h

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
 #ifndef _COMMON_VERSION_H
 #define _COMMON_VERSION_H
 

src/common/keymap.c

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
 #include <common/keymap.h>
 
 // Prevent failures to compile on AVR

src/common/version.c

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
 #include <common/macro.h>
 
 // Prevent failures to compile on AVR