Doc: add GIT_SSL_NO_VERIFY when QEMU clone fail

[Wenzel]

      GEN     config-all-devices.mak
    ./scripts/git-submodule.sh: failed to update modules
  
    Unable to automatically checkout GIT submodules ' ui/keycodemapdb tests/fp/berkeley-testfloat-3 tests/fp/berkeley-softfloat-3 dtc slirp'.
    If you require use of an alternative GIT binary (for example to
    enable use of a transparent proxy), then please specify it by
    running configure by with the '--with-git' argument. e.g.
  
     $ ./configure --with-git='tsocks git'
  
    Alternatively you may disable automatic GIT submodule checkout
    with:
  
     $ ./configure --disable-git-update
  
    and then manually update submodules prior to running make, with:
  
     $ scripts/git-submodule.sh update  ui/keycodemapdb tests/fp/berkeley-testfloat-3 tests/fp/berkeley-softfloat-3 dtc slirp

Can be solved via Export GIT_SSL_NO_VERIFY="true"