boot/grub2: Fix GRUB i386-pc build with Ubuntu gcc

Backport a patch from upstream to fix the build on certain versions of
gsc, notably:

    Ubuntu 19.10 with gcc (Ubuntu 8.3.0-26ubuntu1~19.10) 8.3.0
    Ubuntu 19.10 with gcc (Ubuntu 9.2.1-9ubuntu2) 9.2.1 20191008

The upstream patch is simply a change in the gentpl.py script, which is
used to generate parts of the automake machinery, so if we just backport
the upstream patch, we need to call the script to regenerate those files.

However, the modified script is a python script, so we would need to add
a dependency on host-python (2 or 3), which is not so nice.

Furthermore, calling the script is not enough: it needs a specific set
of optionss for each file it is to generate. That set of options is not
static; it is constructed in the convoluted autogen.sh. Calling
autogen.sh is usally not so good an idea in the Buildroot context, and
indeed this fails becasue it calls to autoreconf, but without our
carefuly crafted options and environment variables.

There was a little light in the tunnel, in that autogen.sh can be told
not to run autoreconf, by setting the environemnt variable
FROM_BOOTSTRAP to an non-=empty string, but this is fraught with various
other side-effects, as in that cause, autogen.sh expects to be valled by
an upper sciopt, bootstrap, which is not provided in the tarball
distribution...

So, between all those issues, autogen, bootstrap, and a host-python (2
or 3) dependency, we choose another route: path the script *and* the one
generated file affected by the change. Since that patched file is a .am
file, we also patch the corresponding .in file

However, we're faced with another issue: the other generated file is
now older than the script, so the automake machinery will now want to
re-run autoconf et al during the build step, which is still not a good
idea for us. So we touch the other generated file so it is mopre recent
than the script.

This is still not sufficient, because the patched file also has a
dependency on the generated file, so we need to touch as well.

Fixes:
 - https://bugs.buildroot.org/show_bug.cgi?id=12946

Signed-off-by: Fabrice Fontaine <[email protected]>
[[email protected]:
  - keep the hunk about patching gentpl.py
  - make it a git-formatted patch
  - add the touch
  - drastically expand the commit log
]
Signed-off-by: Yann E. MORIN <[email protected]>
(cherry picked from commit 7e64a050fbd9add07ed84d48054ffee1b659d079)
Signed-off-by: Peter Korsgaard <[email protected]>