llext-edk: fix: add compile definitions to the generated EDK

The LLEXT EDK was not exporting common Zephyr compile definitions
("-Dxxx" flags). This patch adds the compile definitions before the
other compile flags, as it is done in the Zephyr build system.

This patch also adds to this list the "-DLL_EXTENSION_BUILD" flag,
instead of providing a special case at a later stage.

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