Miscellaneous cleanups/fixes and updates

expat: Use generic download link instead of a hardcoded link to a specific mirror
libpng: Corrected build script
pixman: Fixed hardcoded include and lib paths
squirrel: Removed leftover patch
zlib: Updated from 1.2.5 to 1.2.8
bzip2: Updated from 1.0.4 to 1.0.6

libpng, zlib and bzip2 now download and patch their sources instead of using the pre-patched sources from psp-ports. This makes packages and their patches much easier to maintain. Due to this, the psp-port sources for libpng, zlib and bzip2 are no longer needed and can be removed.

All modified libraries built successfully. I'm unable to test libraries since I do not have any test applications.
I'm pretty confident all of these changes are safe. If a library was working before, then it's probably still working; if a library was broken before, then it's probably still broken.