Add missing hardware headers for gingerbread

This allows building most of the iomx hwbuffer code for gingerbread
as well, even though it's not hooked up completely yet.
Add lists of symbols of libcutils and libui
libiomx: Build the hwbuffer code if found

The new iomx_hwbuffer.c file currently only exists in the master
branch in vlc.git, not in the 2.2 release branch.

Linking the libiomx-*.so to libcutils/liblog/libui even though we
don't use any functions from those (if iomx_hwbuffer.c isn't found)
doesn't hurt, and some of them are necessary even if iomx_hwbuffer.c
isn't built, since we still provide ANDROID_API to iomx.cpp (which
causes the HAS_USE_BUFFER codepaths to be built).
Enable iomx direct rendering if "full acceleration" is chosen

The "auto" option doesn't enable any acceleration at all until
4.3, so this is quite safe - only people who intentionally enable
it will get it. (And for older vlc.git versions, such as the
vlc-2.2 branch, it won't have any effect.)
Add android private headers for jbmr2 and kitkat
libiomx: Add disabled rules for building libiomx for jb* and kitkat

This is useful for debugging.

Don't warn if unable to load these debug libs.
Add android-headers-jb
libiomx: Add disabled build rules for building a separate libiomx for 4.1 and 4.2

This doesn't seem to be necessary in practice - direct rendering with
iomx seems to work just fine with libiomx-ics on both 4.1 and 4.2.