shippable: do not initialize submodules automatically

instead do it in the 'ci' target when needed.

for mips64el-softmmu target:
use dtc submodule if distrib packages are too old.

example with outdated libfdt on mips64el-softmmu target (required is >= 1.4.2):
 # dpkg-query --showformat='${Version}\n' --show libfdt-dev
 1.4.0+dfsg-1

shippable output:
----------------
  LINK    mips64el-softmmu/qemu-system-mips64el
../hw/core/loader-fit.o: In function `load_fit':
/root/src/github.com/philmd/qemu/hw/core/loader-fit.c:278: undefined reference to `fdt_first_subnode'
/root/src/github.com/philmd/qemu/hw/core/loader-fit.c:286: undefined reference to `fdt_next_subnode'
/root/src/github.com/philmd/qemu/hw/core/loader-fit.c:277: undefined reference to `fdt_first_subnode'
collect2: error: ld returned 1 exit status
Makefile:201: recipe for target 'qemu-system-mips64el' failed
make[1]: *** [qemu-system-mips64el] Error 1
Makefile:327: recipe for target 'subdir-mips64el-softmmu' failed
make: *** [subdir-mips64el-softmmu] Error 2

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

.shippable.yml

@@ -1,4 +1,6 @@
 language: c
+git:
+   submodules: false
 env:
   global:
     - LC_ALL=C
@@ -19,5 +21,13 @@ build:
     options: "-e HOME=/root"
   ci:
     - unset CC
+    # some targets require newer up to date packages, for example TARGET_LIST matching
+    # aarch64*-softmmu|arm*-softmmu|ppc*-softmmu|microblaze*-softmmu|mips64el-softmmu)
+    # see the configure script:
+    #    error_exit "DTC (libfdt) version >= 1.4.2 not present. Your options:"
+    #    "  (1) Preferred: Install the DTC (libfdt) devel package"
+    #    "  (2) Fetch the DTC submodule, using:"
+    #    "      git submodule update --init dtc"
+    - dpkg --compare-versions `dpkg-query --showformat='${Version}' --show libfdt-dev` ge 1.4.2 || git submodule update --init dtc
     - ./configure ${QEMU_CONFIGURE_OPTS} --target-list=${TARGET_LIST}
     - make -j$(($(getconf _NPROCESSORS_ONLN) + 1))