Revamped TFTP api for multiple concurrent transfers
Revamped TFTP example to use multiple concurrent transfers
Implements TFTP multiple concurrent transfers (Barebox community request #183)
autotest adaptation to new TFTP syntax
adaptation to new TFTP syntax
adaptation to new TFTP syntax
Moved UDP proto check in dhcp_{client,server} hdrs

Moved after include guards
Duplicated code reduction
Improved readability
Test case addition
Reduced code complexity
Increased code coverage
Minor typo corrections
Addition of RFC 1350