AP_Math: Add missing constexpr

[patrickelectric]

Signed-off-by: Patrick José Pereira patrickelectric@gmail.com

[WickedShell]

@patrickelectric do you get any changes in the resulting build from this? I'm guessing not, but it'd be an interesting note here.

[patrickelectric]

The patch only adds constexpr keyword to allow these functions to run at compile and runtime, the results of the functions does not change.

[WickedShell]

I realize that, I was curious if it actually has helped the compiler at all, or if its just a no op change for being more verbose.

[patrickelectric]

@WickedShell I just check the binary and saw that it does not change the size but it changes the binary content.
diff ardusub_with_constexpr_objdumṕ ardusub_without_constexpr_objdumṕ
http://ix.io/2dyh
(objdump output of constexpr is 210 bytes smaller)

This patch also empowers future patchs that want to create functions with constexpr that uses the ones in this patch.