cmdline: add function to verify valid commands

cmdline: add function to verify valid commands

The cmdline library cmdline_parse() function parses a command and
executes the action automatically too. The cmdline_valid_buffer function
also uses this function to validate commands, meaning that there is no
function to validate a command as ok without executing it.

To fix this omission, we extract the body of cmdline_parse into a new
static inline function with an extra parameter to indicate whether the
action should be performed or not. Then we create two wrappers around
that - a replacement for the existing cmdline_parse function where the
extra parameter is "true" to execute the command, and a new function
"cmdline_parse_check" which passes the parameter as "false" to perform
cmdline validation only.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Tested-by: Weiyuan Li  <weiyuanx.li@intel.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>