neon: replace 'choice' for XML library with two options

The NEON library can either be compiled without XML support, with XML
support provided by Expat, or with XML support provided by
libxml2. Until now, to represent this, a Kconfig 'choice..endchoice'
was used. Unfortunately, another package cannot 'select' one of the
possible choices. So for example, a package such as 'rpm', or the
to-be-added 'subversion' package could not select their dependencies,
they had to do a 'depends on !BR2_PACKAGE_NEON_NOXML', which is not
how Buildroot handles library dependencies in general.

So, this commit replaces the 'choice...endchoice' block with simply
two configuration options that are mutually exclusive. The option
names are not changed, so no Config.in.legacy addition is needed.

An hidden option BR2_PACKAGE_NEON_XML is provided, so that packages
that need XML support in NEON but don't care whether it's provided by
Expat or libxml2 can simply select BR2_PACKAGE_NEON_XML.

The rpm package is updated accordingly.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>