drop support for eglibc version trunk/HEAD

When selecting a specific eglibc version, it comes with a specific SVN
revision that should not be modified as it (more or less) correspond to
a tagged release. This patch disable the possibility to select a specific
SVN revision on known eglib versions.

This patch also disables the possibility to select the trunk branch of
eglibc. There are multiple reasons for that:

* trunk/HEAD may not even compile

* the OpenWrt built system makes using trunk/HEAD a difficult thing, as
OpenWRT fetches the source tree and store it in a compressed tar archive.
Subsequent build get the source from the tar archive - not from SVN,
making the use of trunk/HEAD largelly innefective.

* we cannot know the corresponding version of trunk/HEAD, meaning that
we'll face compiling issues when we'll try to copy the libc files -
unless the build system is fixed with this specific issue in mind.

Signed-off-by: Emmanuel Deloget <logout@free.fr>

SVN-Revision: 31502