Rename libldpc_t2 to libldpc_aal

Rename the library to reflect that it's more a generic AAL driver than
specifically for AAL. To this end, rename some documentation.

As of this commit, the options are still called nrLDPC_coding_t2. The
next commit will correct this.

Co-authored-by: Xin Zhe Khooi <khooixz@comp.nus.edu.sg>

doc/BUILD.md

@@ -185,7 +185,7 @@ Some libraries have further dependencies and might not build on every system:
 - `enbscope`, `uescope`, `nrscope`: libforms/X
 - `ldpc_cuda`: CUDA
 - `websrv`: npm and others
-- `ldpc_t2`: DPDK with patch
+- `ldpc_aal`: DPDK with patch
 
 # Running `cmake` directly