yocto-bsp: update the help regarding the meta-intel layer

yocto-bsp: update the help regarding the meta-intel layer

With move of ia32-base to oe-core, the only case remaining where
meta-intel needs to be added to bblayers.conf for a new BSP is the
case of an x86 BSP that selects EMGD.

Update the documentation to note that fact.

(From meta-yocto rev: b63c199c716d68147def036eb06481245e595802)

Signed-off-by: Tom Zanussi <tom.zanussi@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>