bootspec: normalize function names/parameter lists

bootspec: normalize function names/parameter lists

This normalizes naming of functions operating on BootConfig objects.
Let's always call them boot_config_xyz(), like our usual way to name
stuff.

moreover, move the BootConfig parameter to the beginning, as it's not a
return value (which we typically move to the end of the parameter list),
but simply an object, that even happens to be initialized already.

With these changes the functions are more like our usual way to call
things, and less surprises are good.