Fixed make/firmware-defs.mk to use PYTHON env variable

It caused build errors on systems where python points to python3.

Signed-off-by: Francesco Nero <nerofrancesco@hotmail.com>
Removed redefinition of PYTHON var in makefiles

Also fixed some typos in some makefiles.

Signed-off-by: Francesco Nero <nerofrancesco@hotmail.com>
Merge pull request #1129 from francesconero/next

Fixed make/firmware-defs.mk to use PYTHON env variable