llext-edk: fix -imacros handling

The -imacros flag may be given in a number of different variations:
 - with one or two preceding dashes, and
 - separated from its argument, joined by an equals sign, or joined with
   no separator

This patch fixes the handling of the -imacros flag in the LLEXT EDK to
detect all of those options.

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