uname: Fixed calls to uname to use busybox

Not all devices have uname, since devlib deploys its own busybox binary
the most portable way to use uname is to use it via busybox.

devlib/target.py

@@ -82,7 +82,7 @@ class Target(object):
     @property
     @memoized
     def kernel_version(self):
-        return KernelVersion(self.execute('uname -r -v').strip())
+        return KernelVersion(self.execute('{} uname -r -v'.format(self.busybox)).strip())
 
     @property
     def os_version(self):  # pylint: disable=no-self-use
@@ -515,7 +515,7 @@ class LinuxTarget(Target):
     @property
     @memoized
     def abi(self):
-        value = self.execute('uname -m').strip()
+        value = self.execute('{} uname -m'.format(self.busybox)).strip()
         for abi, architectures in ABI_MAP.iteritems():
             if value in architectures:
                 result = abi