Meson is picky about the arguments of build options.

For example a option of the "boolean" type only accepts true/false this
option is covered by ${opt}_MESON_TRUE/_FALSE.

Add option helpers ${opt}_MESON_YES/_NO for the "combo" type which
accepts yes and no.

Approved by: portmgr@ (mat@)
Differential Revision: https://reviews.freebsd.org/D11078