xerces: fix dependency on iconv

All "select BR2_PACKAGE_LIBICONV" must use the "if !BR2_ENABLE_LOCALE"
condition, otherwise we can end up with a toolchain suppoting locales
*and* the libiconv package being compiled, which confuses other
packages. Example with glib:

gconvert.c:52:2: error: #error GNU libiconv in use but included
iconv.h not from libiconv

In addition to that, in xerces.mk, we add the dependency on libiconv
when it is available, to make sure it gets compiled before xerces.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>