allarch, packagegroups: Fix different signatures for allarch recipes

:Release Notes:
Couple of variables are already set to empty in allarch.bbclass, but
TARGET_PREFIX isn't because typicaly TARGET_PREFIX is set by DISTRO
and the same for all MACHINE-s. We need to inhibit all dependencies
even when packagegroup isn't allarch. Always inhibit default
dependendies, because things like virtual/${TARGET_PREFIX}gcc don't
exist when TARGET_PREFIX is empty.

:Detailed Notes:
External toolchain usually need different TARGET_PREFIX and when you're
building for 2 MACHINEs with different tcmode it was causing different
signatures for allarch recipes. Also fix cracklib-native,
packagegroup-core-ssh-dropbear, packagegroup-core-boot.

:Testing Performed:
Only build tested and verified with sstate-diff-machines.sh.

:QA Notes:
No change in image.

:Issues Addressed:
[GF-44050] allarch components have different signatures with different
           tcmode

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@lge.com>
Change-Id: I795cccaea656fd33e7262237bd04f876e5c6371e
Reviewed-on: https://g2g.palm.com/3744
Reviewed-by: DCO Verification
Reviewed-by: Susan Montooth <susan.montooth@lge.com>
Tested-by: Susan Montooth <susan.montooth@lge.com>