psplash: Add mising commands module import.

psplash: Add mising commands module import.

Python commands module is used in the recipe, but never imported.
This leads to build errors like:

NameError: global name 'commands' is not defined

ERROR: The stack trace of python calls that resulted in this exception/failure was:
ERROR:   File "do_compile", line 24, in <module>
ERROR:
ERROR:   File "do_compile", line 11, in do_compile
ERROR:
ERROR: The code that was being executed was:
ERROR:      0020:        bb.build.exec_func("oe_runmake", d)
ERROR:      0021:        shutil.copyfile("psplash", outputfile)
ERROR:      0022:
ERROR:      0023:
ERROR:  *** 0024:do_compile(d)
ERROR:      0025:
ERROR: (file: 'do_compile', lineno: 24, function: <module>)
ERROR:      0007:    localfiles = d.getVar('SPLASH_LOCALPATHS', True).split()
ERROR:      0008:    outputfiles = d.getVar('SPLASH_INSTALL', True).split()
ERROR:      0009:    for localfile, outputfile in zip(localfiles, outputfiles):
ERROR:      0010:        if localfile.endswith(".png"):
ERROR:  *** 0011:            outp = commands.getstatusoutput('./make-image-header.sh %s POKY' % localfile)
ERROR:      0012:            print(outp[1])
ERROR:      0013:            fbase = os.path.splitext(os.path.basename(localfile))[0]
ERROR:      0014:            shutil.copyfile("%s-img.h" % fbase, destfile)
ERROR:      0015:        else:

(From OE-Core rev: 34bdb9a459208377210454e54635afb9404ee126)

Signed-off-by: Peter Tworek <tworaz666@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>