openssl: fix pkgconfig path problems

openssl: fix pkgconfig path problems

OpenSSL 3.3.0 introduced new pkgconfig generators which interact badly
with our incorrect setting of --libdir, which is documented as being
the name of the directory _under $prefix_, not an absolute path.  This
resulted in the pkgconfig files have libdir=/usr which mostly works as
the actual library directory is on the search path, but can break other
recipes (such as tpm2-openssl).

Pass the correct value for --libdir, and also remove the odd handling of
an empty ${prefix} which is very historical[1] and can't happen anymore
as all build variations have a prefix.

[1] Added in oe-classic f725a81c, 2009

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>