try make travis build pass

.travis.yml

@@ -1,11 +1,16 @@
 language: c
-dist: trusty
+dist: bionic
 sudo: false
 addons:
   apt:
     packages:
       - libffi-dev
       - pkg-config
+      - bison
+      - flex
+      - build-essentials
+      - autoconf
+      - automake
 
 
 script:
@@ -17,6 +22,9 @@ script:
   - echo -e "travis_fold:start:build_micropython"
   - echo "Building micropython"
   - git clone --recursive https://github.com/micropython/micropython.git
+  - pushd micropython/mpy-cross
+  - make
+  - popd
   - pushd micropython/ports/unix
   - git describe --always --tags
   - make axtls
@@ -53,5 +61,6 @@ script:
   - echo -e "travis_fold:end:compat_tests"
 
   - echo -e "travis_fold:start:rtc_macro_examples"
+  - ln -s ../binutils-esp32ulp  # already cloned earlier. reuse.
   - ./02_compat_rtc_tests.sh
   - echo -e "travis_fold:end:rtc_macro_examples"