package/rpi-firmware: add option to install vcdbg

vcdbg is a tool to help debug the communication with the GPU.
It comes as a binary-only, and in two flavours: one for the hard
floating point ABI, one for the software floating point ABI.

Unfortunately, we have no source code for that tool, only a binary that
was dynamically linked with glibc and libraries from rpi-userland.

So, just install that executable, and let's hope there is no symbol
issue at runtime.

Note: vcdbg needs glibc, threads and !static. Since glibc already
implies threads and !static, we only need to depend on glibc.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc; Floris Bos <bos@je-eigen-domein.nl>
Cc: Pascal de Bruijn <pmjdebruijn@pcode.nl>
Cc: Baruch Siach <baruch@tkos.co.il>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>