oeqa/selftest/gcc: Fix host key verfication failure

oeqa/selftest/gcc: Fix host key verfication failure

while runnig oe-selftest for gcc, testcases that need to be run on qemu are not running due to below failures.
- Executing on ssh: mkdir -p /tmp/runtest.3549641   (timeout = 300)
   spawn [open ...]
   Host key verification failed.
   ERROR: Couldn't create remote directory /tmp/runtest.3549641 on ssh

Host key verification failure is happening when ssh board config file name is
defined as "ssh.exp" and there are multiple ssh.exp files generated during the build
and a wrong ssh config was taken. To resolve this changed the board config file name
to "linux-ssh.exp" which ensures correct ssh settings are used.

Signed-off-by: Harish Sadineni <Harish.Sadineni@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>