* [Bugfix][VTA] PkgConfig cause crash in PYNQ board due to link library
not exist.

Symptom:
When run vta_get_started.py with pynq board, host crash and
complain "cannot find -lsds_lib" and "cannot find -l:libdma.so"

Reproduce:
At pynq board, delete the ./build/vta_config.json, then run rpc
server.
In host machine run vta_get_started.py, issue would reproduce.

Analysis:
This issue caused by 'PkgConfig' function  still using pynq2.1
library which not exist in pynq2.4 anymore, when a "reconfig_runtime"
logic of rpc_server.py get triggered , the compile would failed due to
link library not exist.

Solution:
change the link library to libcma.so.

* [Document Change][VTA] Change pynq version from 2.3 into 2.4.

Issue:
pynq 2.3 image not available anymore from pynq download page and pynq
2.4 is the current latest image which available in the said website, after
verification, currently VTA work good with pynq 2.4 image, hence update
related document from pynq 2.3 to 2.4.