Simplify defaulting of ARCH and FQBN

bin/kaleidoscope-builder

@@ -424,13 +424,10 @@ do_compile () {
         # shellcheck disable=SC1090
         FQBN="$(. "${SKETCH_DIR}"/.kaleidoscope-builder.conf && echo "${FQBN}")"
         if [ -n "${BOARD}" ]; then
-            if [ -z "${ARCH}" ]; then
+            : "${ARCH:=avr}"
-               FQBN="keyboardio:avr:${BOARD}"
-            else
             FQBN="keyboardio:${ARCH}:${BOARD}"
         fi
     fi
-    fi
 
     _CMD_CXX="${CXX:-${COMPILER_PREFIX}${CXX_COMPILER_BASENAME}${COMPILER_SUFFIX}}"
     _CMD_CC="${CC:-${COMPILER_PREFIX}${C_COMPILER_BASENAME}${COMPILER_SUFFIX}}"