logger: add git branch name to the log

src/lib/version/version.h

@@ -118,6 +118,12 @@ __EXPORT const char *px4_toolchain_version(void);
  */
 __EXPORT const char *px4_firmware_version_string(void);
 
+/**
+ * get the git branch name (can be empty, for example if HEAD points to a tag)
+ */
+__EXPORT const char *px4_firmware_git_branch(void);
+
+
 /**
  * Firmware version in binary form (first part of the git tag)
  */