siggen: Fix shared work checksum mismatch/rebuild issues

Similar to the last shared work task signature bug, we've found another
one. Looking at the improved output of diffsigs in this case:

runtaskdeps changed from [
'autoconf_2.69.bb.do_populate_sysroot:virtual:native',
'gnu-config_20120814.bb.do_populate_sysroot:virtual:native',
'libgcc-initial_4.9.bb.do_patch:virtual:nativesdk'
] to [
'autoconf_2.69.bb.do_populate_sysroot:virtual:native',
'gcc-crosssdk-initial_4.9.bb.do_patch',
'gnu-config_20120814.bb.do_populate_sysroot:virtual:native'
]

so we can get a different task hash since libgcc sorts before gnu-config
and gcc sorts after it. We could do with a way of fixing this, the best
I can come up with is to include a single parent directory. Since
recipes are never at the top of any metadata trees I've seen, this
should suffice for now.

I'm planning to burn the concept of shared work within bitbake
and do something at the metadata level in the 1.8 timeframe as its just
too fragile as things stand and hard to fix well.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>