common: drop 0/no logic from XBPS_{CHECK_PKGS,BUILD_ONLY_ONE_PKG}

use the common convention as described in etc/defaults.conf

common/xbps-src/libexec/xbps-src-docheck.sh

@@ -27,7 +27,7 @@ if [ -n "$XBPS_CROSS_BUILD" ]; then
     exit 0
 fi
 
-if [ -z "$XBPS_CHECK_PKGS" -o "$XBPS_CHECK_PKGS" = "0" -o "$XBPS_CHECK_PKGS" = "no" ]; then
+if [ -z "$XBPS_CHECK_PKGS" ]; then
     msg_normal "${pkgname}-${version}_${revision}: skipping check (XBPS_CHECK_PKGS is disabled) ...\n"
     exit 0
 fi