Support for embedding arbitrary string to Linux kernel command line

Issue #45

This commit adds supports for adding arbitrary strings to the Linux
kernel commandline.

A user can set a shell env variable EXTRA_CMDLINE_LINUX with a
string that needs to be embedded in the kernel commandline.

Exmaple usage provided in machine/kvm_x86_64/machine.make.

NOTE: Currently, we're only supporting x86 arch.