build: abstract emulation and replace qemu goal with run

This will replace the current goal of 'make qemu' with 'make run' and
moves Qemu handling into its own file and into the boards instead of
being architecture specific.

We should be able to add new boards that support some other type of
emulation (by adding scripts/Makefile.<emu type>) and allow the board to
define their own options for the use type of emulation.

'make qemu' will still work, however it will be deprecated, starting
with this commit it is recommended to use 'make run'.

Jira: ZEP-359
Change-Id: I1cacd56b4ec09421a58cf5d010e22e9035214df6
Signed-off-by: Anas Nashif <anas.nashif@intel.com>