hob: make some layers non removable

hob: make some layers non removable

- there are some layers which cannot be removed; so ,I have used
a new variable called BBLAYERS_NON_REMOVABLE located in bblayers.conf,
which contains those layers which cannot be deleted
- "meta-hob" layer is added to this variable in hob code, like it's
added to BBLAYERS var

[YOCTO #3176]
Signed-off-by: Cristiana Voicu <cristiana.voicu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>