This commit introduces a new options for buildrump.sh, which is '-l'
option to specify which rumpkernel (netbsd or linux) is used (default:
netbsd).  Linux kernel is based on Linux Kernel Library (LKL) with
non-upstreamed patches to implement with rump hypercall interfaces.

Since LKL is not yet upstreamed to Linus tree and development is still
ongoing, the commit tries to minimize future modifications to this
script (buildrump.sh) by specifying interfaces to build Linux kernel.

The new rumpkernel support involves the script structure refactor to
split each rumpkernel dependent commands into separate files (netbsd.sh
and linux.sh).

Currently, when '-l' option is specified, NetBSD rump kernel code will
be also built in addition to Linux kernel build since Linux part
requires headers files (i.e., include/rump) to build it.

A few more travis tests are also added though it's only build tests and
no execution tests for Linux at this moment though.  Plus, clang build
is marked as 'allow_failure' since Linux kernel isn't able to build with
clang so far.

There are still a couple of limitations:
- LKL_SRCDIR is hardcoded, which uses the current dir of the (root)
script.
- only build with standalone buildrump.sh.  standalone execution (i.e.,
without rumprun or frankenlibc) are not implemented since LKL itself has
to do it.
- thus, no standalone test are implemented either.

Signed-off-by: Hajime Tazaki <thehajime@gmail.com>