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example with outdated libfdt on mips64el-softmmu target (required is 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>
