buildtools: allow string constant padding

buildtools: allow string constant padding

Size of string constant symbol may be larger than its length
measured up to NUL terminator. In this case pmdinfogen included padding
bytes after NUL terminator in generated source, yielding incorrect code.

Always trim string data to NUL terminator while reading ELF.
It was already done for COFF because there's no symbol size.

Bugzilla ID: 720
Fixes: f0f93a7adfee ("buildtools: use Python pmdinfogen")
Cc: stable@dpdk.org
Signed-off-by: Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>