samples: get rid of obsolete usage of ARCH variable

Using ARCH variable to select different configurations for the different
architectures is misleading and conflicts with the variable ARCH being used by
the build system.  The variable is not needed, it is application specific and
an application can be built without the need to specify ARCH on the command
line.

This is yet another item specific to samples and test cases that
wrongfully being used and documented for every application.  We need to use
another variable and just make it clear it is specific to samples and how they
are written.  One possible solution is to have a script that gets the
architecture based on the board being used.  Attachments

Jira: ZEP-238
Change-Id: Ieccbc087a41858fb96fb361c0aaa04705e968a4e
Signed-off-by: Anas Nashif <anas.nashif@intel.com>