scripts: consolidate code to find bitbake path

Several of these scripts were using duplicated code (and slightly
different methods) to find the path to bitbake and add its lib
subdirectory to the Python import path. Add some common code to do this
and change the scripts to use it.

Fixes [YOCTO #5076].

(From OE-Core rev: 0b5e94e168819134dcda0433c8ae893df4ab13ce)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/bitbake-whatchanged

@@ -27,18 +27,18 @@ import warnings
 import subprocess
 from optparse import OptionParser
 
+scripts_path = os.path.abspath(os.path.dirname(os.path.abspath(sys.argv[0])))
+lib_path = scripts_path + '/lib'
+sys.path = sys.path + [lib_path]
+
+import scriptpath
+
 # Figure out where is the bitbake/lib/bb since we need bb.siggen and bb.process
-p = subprocess.Popen("bash -c 'echo $(dirname $(which bitbake-diffsigs | grep -v \'^alias\'))/../lib'",
-        shell=True, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
-
-err = p.stderr.read()
-if err:
-    print("ERROR: Failed to locate bitbake-diffsigs:", file=sys.stderr)
-    print(err, file=sys.stderr)
+bitbakepath = scriptpath.add_bitbake_lib_path()
+if not bitbakepath:
+    sys.stderr.write("Unable to find bitbake by searching parent directory of this script or PATH\n")
     sys.exit(1)
 
-sys.path.insert(0, p.stdout.read().rstrip('\n'))
-
 import bb.siggen
 import bb.process