Update git submodules

* Update external/lk from branch 'master'
  to 6682011fa829649000e5587ce02559e069e38e3f
  - [lib][debug] Fix UBSan issue
    
    Use unsigned char to replace char to avoid potential conversion overflow
    
    Bug: 119111590
    Change-Id: I1d70d9acd371a8c9877ae76bfe82a74c9dac7cd9
    
1 file changed