Only import bz2 python module when needed

The js2c.py script imports the bz2 module unconditionnally, which
would require us to build the bzip2 support in host-python. Since in
fact bzip2 support is not technically used when building this package,
we ensure that the bz2 module is only imported when needed.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/src/3rdparty/v8/tools/js2c.py
===================================================================
--- a/src/3rdparty/v8/tools/js2c.py
+++ b/src/3rdparty/v8/tools/js2c.py
@@ -33,7 +33,6 @@
 
 import os, re, sys, string
 import jsmin
-import bz2
 
 
 def ToCAsciiArray(lines):
@@ -344,6 +343,7 @@
   else:
     raw_sources_declaration = RAW_SOURCES_COMPRESSION_DECLARATION
     if env['COMPRESSION'] == 'bz2':
+      import bz2
       all_sources = bz2.compress("".join(all_sources))
     total_length = len(all_sources)
     sources_data = ToCArray(all_sources)