qt: split script and webkit supported arches in two different variables

qt: split script and webkit supported arches in two different variables

Currently in Buildroot we have a BR2_PACKAGE_QT_ARCH_SUPPORTS_WEBKIT
variable indicating which architectures support Qt Webkit. We also make
Qt Script depending on that variable, so we are assuming that Qt Script
is supported for exactly the same architectures which support Qt Webkit,
and that's not true.

For instance, Qt Webkit is not supported for MIPS64 when
using the n32 ABI, but Qt Script is actually supported. So, if we make
BR2_PACKAGE_QT_ARCH_SUPPORTS_WEBKIT depending on !BR2_MIPS_NABI32 we
will also disable Qt Script, because as I said before, Qt Script depends
on BR2_PACKAGE_QT_ARCH_SUPPORTS_WEBKIT, and we don't want that because
Qt Script works.

We fix this by creating another variable called
BR2_PACKAGE_QT_ARCH_SUPPORTS_SCRIPT to state which architectures support
Qt Script, so now we can differentiate them from the ones supporting Qt
Webkit.

Related:
  http://lists.busybox.net/pipermail/buildroot/2014-November/112605.html

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>