distutils: Introduce PYTHON_ABI variable

distutils: Introduce PYTHON_ABI variable

In python3 this has to be taken care of, it defines a variable to
denote ABI currently its at 'm' and it uses this to construct
the directory names for installing the python headers and library
names in sysroot. e.g. it will be something like ../python3.3m/...

We need this information when we are using distutils to cross build
python extentions and want to know the locations of python headers
and libraries install locations

Signed-off-by: Khem Raj <raj.khem@gmail.com>