samples: ipm_esp32: use net core source from build ouput

Current ipm_esp32 sample code requires esp32_net_firmware.c file
as source input, which is generated after ipm_esp32_net build.
esp32_net_firmware.c was manually added as binary blob, which needs to
be removed from the repository.

This change modifies ipm_esp32 CMakeLists.txt to include the build
output of ipm_esp32_net as input source of ipm_esp32 build, allowing the
binary blob to be removed.

Current approach shall be updated during next release.

Signed-off-by: Sylvio Alves <sylvio.alves@espressif.com>