Allow specifying additional qemu arguments for externally supported boards
(e.g. zynq_qemu) by adding a `qemu_args` file in the board-property directory.
The syntax of the qemu_args file is as follows:
- Arguments can appear in a single line or in multiple lines as the
lines will be appended (separated by a whitespace) to the global
qemu_args variable.
- If the line is prepended with a `foobar:` expression. The arguments
are only added if the foobar spec is present.
Note, that a `-m` argument specified in the qemu_args file will
override the arguments provided by the run scripts.
genodelabs/genode#4311
