[buildbot] Customized build and board file

[Yao Qi <qiyaoltc@gmail.com>]

Hi Sergio,
I want to add a new build slave on gcc114 (aarch64) for aarch32
gdb testing, that is, gdb and test cases are built by arm-linux-gnueabihf
compiler.

I want to configure gdb with "--target=arm-linux-gnueabihf
--host=arm-linux-gnueabihf --build=arm-linux-gnueabihf", and other options,
and also provide a board file "unix-aarch32.exp" in which
arm-linux-gnueabihf can be used to compile test cases.

Can we do these things above in our buildbot infrastructure?

Note that this is trade-off among various limitations,
 - we don't have armv7 devices in gcc compile farm, so we have to emulate
   on an armv8 machine,
 - libc6:armhf is not installed on gcc114, so I build the toolchain, and force
   gdb and test programs use my sysroot.  I am going to raise a request to
   install libc6:armhf, but it may take a while.

-- 
Yao (齐尧)