Reduce flash usage by 3.7K by changing CMS data structures

[fiam]

After discussing CMS space optimisations with @hydra, the following changes were identified as potential flash savers:

• Packing OSD_Entry struct. It's not used in performance critical code, so it shouldn't impact flight characteristics.
• Reworking the fields in OSD_Entry so they're naturally aligned (reduces the code needed to deal with the packed struct)
• Combine OSD_Back and OSD_END into a single OSD_BACK_AND_END item, saving an OSD_Entry per menu.

Tested and ready to merge. Could probably wait for 2.2.

[teckel12]

@fiam Nice, keeping the F3 viable a bit longer.

[hydra]

i suspect that you might save even more flash if you drop the back/end marked all together and pass sound a structure for the menu.
something like:

typedef struct cmsMenu_s {
const OSD_Entry entries;
const uint8_t entry Count;
} cmsMenu_t

would save the size of OSDEntry_t - size of cmsMenu_t per menu.

likely the loops become easier and conditional code may be removed too.

[hydra]

Don't let my comment above prevent you from merging this however. You can do another PR for that later.