llext: refactor llext_copy_region()

This commit adjusts the logic of the LLEXT memory allocation routine by:

 * defining 'align' and 'alloc' variables once and updating them in the
   following code;

 * swapping the condition, converting it to an 'if(IS_ENABLED(...))' and
   using a direct calculation for the size to make the code easier to
   read;

 * updating and extending a few comments.

No functional change is intended.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>