libpng12: rename libpng_1.2.50 to libpng12

libpng12: rename libpng_1.2.50 to libpng12

As Mark's suggestion, rename libpng_1.2.50 to libpng12 that
multi-versions libpng could coexist.

We want to make sure we have both the old and new versions to meet LSB
compliance (for people who have that enabled) as well as the new version
for newer applications.

And drop link files that conflict with higher version.

[YOCTO #4221]

(From OE-Core rev: fc626e6861e491b0144b813a5b48b0f5f57664e6)

Signed-off-by: Kang Kai <kai.kang@windriver.com>
CC: Mark Hatle <mark.hatle@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>