base-files: properly fix copying ld*.so with eglibc

r36107 ("base-files: sysupgrade fail with eglibc") tried to fix
sysupgrade by changing the ld-soname to what is expected, but only
fixed MIPS while breaking ARM.

The underlying issue is that the ld.so name varies widely across
different architectures for eglibc:

eglibc-2.19-r25243$ grep -r "ld-soname :=" . | awk '{ print $3 }' | sort -u
ld64.so.1
ld64.so.2
ld-linux-aarch64_be.so.1
ld-linux-aarch64.so.1
ld-linux-armhf.so.3
ld-linux-mipsn8.so.1
ld-linux.so.2
ld-linux.so.3
ld-linux-x32.so.2
ld-linux-x86-64.so.2
ld.so.1

Instead of adding each different soname to check for and copy it,
replace the awk script with a sed script to extract it properly and
drop the hardcoded so-name.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43295 3c298f89-4303-0410-b956-a3cf2f4a3e73