[build] add SPINEL_ENCRYPTER_LIBS option to common switches (#5334)
[build] add SPINEL_ENCRYPTER_LIBS option to common switches (#5334)

This option allows to pass Spinel Encrypter library to predefined
Makefiles for different platforms.

Example usage:
make -f examples/Makefile-nrf52840 SPINEL_ENCRYPTER_LIBS=my_encrypter_lib.a
srickardti/openthread,abtink/openthread,chshu/openthread,openthread/openthread,srickardti/openthread,jwhui/openthread,chshu/openthread,jwhui/openthread,openthread/openthread,bukepo/openthread,openthread/openthread,jwhui/openthread,openthread/openthread,srickardti/openthread,bukepo/openthread,chshu/openthread,abtink/openthread,abtink/openthread,jwhui/openthread,bukepo/openthread,abtink/openthread,chshu/openthread,chshu/openthread,bukepo/openthread,srickardti/openthread,chshu/openthread