buildroot

xenomai: add string option for additional configure options

Xenomai has many configure options that users may or may not want to set.
Providing individual Buildroot config options for every single one of them
is not maintainable.

Therefore, add a string option to allow the needed flexibility.
Important options, or those that have 'select/depends on' impact, can still
be turned into real Buildroot config options.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
[Thomas: rewrap Config.in help text.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>