Python2: don't install 2to3, clashes with Python3 equivalent

[MilhouseVH]

Noticed this Python3 failure on Jenkins:

(cd /var/lib/jenkins/LE/build2/workspace/Addons/Category/service/snapserver/build.LibreELEC-RK3328.arm-9.1-devel/toolchain/bin; ln -s 2to3-3.7 2to3)
/usr/bin/install -c -m 644 /var/lib/jenkins/LE/build2/workspace/Addons/Category/service/snapserver/build.LibreELEC-RK3328.arm-9.1-devel/Python3-3.7.1/Include/fileutils.h /var/lib/jenkins/LE/build2/workspace/Addons/Category/service/snapserver/build.LibreELEC-RK3328.arm-9.1-devel/toolchain/include/python3.7
/usr/bin/install -c -m 644 /var/lib/jenkins/LE/build2/workspace/Addons/Category/service/snapserver/build.LibreELEC-RK3328.arm-9.1-devel/Python3-3.7.1/Include/floatobject.h /var/lib/jenkins/LE/build2/workspace/Addons/Category/service/snapserver/build.LibreELEC-RK3328.arm-9.1-devel/toolchain/include/python3.7
ln: failed to create symbolic link '2to3': File exists
make: *** [Makefile:1225: bininstall] Error 1

Python2 is also installing 2to3 in toolchain/bin:

copying build/scripts-2.7/2to3 -> /var/lib/jenkins/LE/build2/workspace/Addons/Category/service/snapserver/build.LibreELEC-RK3328.arm-9.1-devel/toolchain/bin

so if Python2 finishes before Python3, then Python3 will fail as above.

Obviously we need to prevent Python2 installing toolchain/bin/2to3. I can't find a clean solution so have patched it out for now (only temporary until we bin Python2 completely).

[MilhouseVH]

More fun with Python. Adding the clobber commit.

Python2 creates files with read-only permissions:

neil@nm-linux:/tmp/toolchain$ ls -la /home/neil/projects/LibreELEC.tv/build.LibreELEC-Generic.x86_64-9.1-devel/toolchain/x86_64-libreelec-linux-gnu/sysroot/usr/lib/libpython2.7.so.1.0
-r-xr-xr-x 1 neil neil 3952760 Mar  3 00:43 /home/neil/projects/LibreELEC.tv/build.LibreELEC-Generic.x86_64-9.1-devel/toolchain/x86_64-libreelec-linux-gnu/sysroot/usr/lib/libpython2.7.so.1.0

so a re-build fails because the new files from the isolated sysroot can't overwrite the existing read-only files. Eugh.

-f to the rescue.