lib: json: Simplify lexing of "true", "false", and "null" tokens

Roll the loop in an accept_run() function and use it to match "rue" and
"alse" depending on the first character of the token.  Use that to lex
"ull" after finding "n" as well.  This reduces the code slightly.

Jira: ZEP-1607
Change-Id: Iec8ff6ae2fb79e7fe65d476d1574c5943d23e14f
Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>