siggen: add quotes around variable values to see whitespace

* without this change it's hard to see what's changed:
  bitbake-diffsigs sstate-cache/*/sstate-depmodwrapper-cross-cortexa8-vfp-neon-oe-linux-gnueabi-1.0-r0-cortexa8-vfp-neon-*_populate-sysroot.tgz.siginfo
  basehash changed from 62f28c13172d38af917716d6d4b15ac5 to 9cf38842bcc79e0a4356856a64141548
  Variable TUNE_CCARGS value changed from  -march=armv7-a    -mthumb -mthumb-interwork -mfloat-abi=softfp -mfpu=neon -mtune=cortex-a8  to  -march=armv7-a    -mthumb -mthumb-interwork -mfloat-abi=softfp -mfpu=neon -mtune=cortex-a8
  Hash for dependent task depmodwrapper-cross_1.0.bb.do_install changed from 4494cb6c117309a9b4c541ad86c0aac5 to 1e3bcf2d14e5bd30a38350b41bb98db3

* with this change, we can see extra trailing space in first version:
  basehash changed from 62f28c13172d38af917716d6d4b15ac5 to 9cf38842bcc79e0a4356856a64141548
  Variable TUNE_CCARGS value changed from ' -march=armv7-a    -mthumb -mthumb-interwork -mfloat-abi=softfp -mfpu=neon -mtune=cortex-a8 ' to ' -march=armv7-a    -mthumb -mthumb-interwork -mfloat-abi=softfp -mfpu=neon -mtune=cortex-a8'
  Hash for dependent task depmodwrapper-cross_1.0.bb.do_install changed from 4494cb6c117309a9b4c541ad86c0aac5 to 1e3bcf2d14e5bd30a38350b41bb98db3

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>