git: make expat and curl into PACKAGECONFIG items

It can be useful to use git on target (e.g. with some wrapper like
etckeeper for keeping track of changes to /etc), and for such cases,
it is likely one has no need for pulling from/pushing to http[s]
repositories. From the INSTALL file:

    - "libcurl" library ... If you do not use http:// or https://
      repositories, and do not want to put patches into an IMAP
      mailbox, you do not have to have them (use NO_CURL).

    - "expat" library; git-http-push uses it for remote lock
      management over DAV.  Similar to "curl" above, this is
      optional (with NO_EXPAT).

Setting --without-expat and --without-curl reduces the size of the
installed "git" package from 18M to 12M, in addition to avoiding
pulling those libraries into the rootfs.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>