lualoader: allow graphical bits to be disabled with loader_gfx

[kevans]

Summary

Some people prefer the old ASCII art look and it's good to have a way
to confirm that the fallbacks still work right on systems that have a
functional framebuffer available. Add a loader_gfx loader.conf(5)
variable to disable the eager use of graphics for these use-cases.

While we're here, clean up the style in the area a little bit; the early
porting that I did to lualoader did a lot of redundant ~= nil that has
carried over into some of the later work. We can drop some of that, and
also re-organize some of these variables to improve readability.

[kevans]

Rename to loader_gfx, cover the brand and menu frame as well

[ziaee]

{F119160589}

Tested by: ziaee

{F119160774}

Although the offset of the orb is a bit changed due to centering it for the graphical logo a few months ago.

[kevans]

In D50706#1157524, @ziaee wrote:

{F119160589}

Tested by: ziaee

{F119160774}

Although the offset of the orb is a bit changed due to centering it for the graphical logo a few months ago.

18:13 <@kevans91> the graphical bits in gfx-*.lua should've been a gfx { image, shift, height } table or 
                  something
[...]
18:22 <@kevans91> maybe if i get bored on a flight next week i'll play with it