Doc: add GIT_SSL_NO_VERIFY when QEMU clone fail

~~~
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"`