basic/efivars: replace dynanamic creation of efivar names with static strings

basic/efivars: replace dynanamic creation of efivar names with static strings

Creating those string dynamically at runtime is slow and unnecessary.
Let's use static strings with a bit of macro magic and the let the compiler
coalesce as much as possible.

$ size build/src/shared/libsystemd-shared-248.so{.old,}
   text	   data	    bss	    dec	    hex	filename
2813453	  94572	   4584	2912609	 2c7161	build/src/shared/libsystemd-shared-248.so.old
2812309	  94564	   4584	2911457	 2c6ce1	build/src/shared/libsystemd-shared-248.so

A nice side-effect is that the same form is used everywhere, so it's easier to
figure out all variables that are used, and where each specific variable is
used.

C.f. 2b0445262ad9be2a9bf49956ab8e886ea2e48a0a.

Note: 'const char *foo = alloca(…);' seems OK. Our coding style document and
alloca(3) only warn against using alloca() in function invocations. Declaring
both stack variable and alloca at the same time should be fine: no matter in
which order they happen, i.e. if the pointer variable is above the contents,
or the contents are above the pointer, or even if the pointer is elided by the
compiler, everything should be fine.