buildhistory.bbclass: Specify lang in decoding strings

buildhistory.bbclass: Specify lang in decoding strings

On systems where default locale is utf-8 we get errors like

File: 'buildhistory.bbclass', lineno: 38, function: write_pkghistory
0034: if pkginfo.rconflicts:
0035: f.write("RCONFLICTS = %s\n" % pkginfo.rconflicts)
0036: f.write("PKGSIZE = %d\n" % pkginfo.size)
0037: f.write("FILES = %s\n" % pkginfo.files)
*** 0038: f.write("FILELIST = %s\n" % pkginfo.filelist)
0039:
0040: for filevar in pkginfo.filevars:
0041: filevarpath = os.path.join(pkgpath, "latest.%s" % filevar)
0042: val = pkginfo.filevars[filevar]
Exception: UnicodeEncodeError: 'ascii' codec can't encode character
u'\xed' in position 337: ordinal not in range(128)

This patch specifies decode to use utf-8 so ascii and utf-8 based
locales both work

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>