Build System: Prevent unneeded changes to asterisk/buildopts.h.
* Add AST_DEVMODE to BUILDOPTS
* Use BUILDOPTS to generate AST_BUILDOPT_SUM.
* Remove loop that defined AST_MODULE_*
These changes ensure that only ABI effecting options are considered for
AST_BUILDOPT_SUM. This also reduces unneeded full system rebuilds caused
by enabling or disabling one module that another is dependent on.