package/qemu: add BR2_PACKAGE_HOST_QEMU_USER_MODE_ARGS string

For specific architectures, running qemu in user mode without any additional
options may fail if the host processor does not have the necessary instructions
to properly run qemu in user mode, which results in the following error:
"qemu: uncaught target signal 4 (Illegal instruction) - core dumped"

CoreI7 is one such architecture that has had consistent auto-build failures.

Add a new string in qemu/Config.in.host: BR2_PACKAGE_HOST_QEMU_USER_MODE_ARGS.
The default for the Corei7 architecture is directly from the OpenEmbedded
project found in meta/conf/machine/include/x86/tune-corei7.inc:
"-cpu Nehalem,check=false." Other architectures may be added to this string at
a later date if other failures occure.

Signed-off-by: Adam Duskett <aduskett@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/qemu/Config.in.host

@@ -73,6 +73,10 @@ config BR2_PACKAGE_HOST_QEMU_LINUX_USER_MODE
 	  the running host kernel, you may run into invalid system
 	  calls, which may yield surprising effects.
 
+config BR2_PACKAGE_HOST_QEMU_USER_MODE_ARGS
+	string
+	default "-cpu Nehalem,check=false" if BR2_x86_corei7
+
 config BR2_PACKAGE_HOST_QEMU_VDE2
 	bool "VDE2 support"
 	help