llext-edk: (refactor) unify variable names

The variable llext_edk_cflags was used in the main CMakeLists.txt file,
while llext_cflags was used in the llext-edk.cmake file. This commit
unifies the variable names to use llext_edk_cflags in both files.

No logic changes are introduced by this commit.

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