arc: make AXS101 default platform

arc: make AXS101 default platform

This fixes building in automated flow that doesn't use defconfigs.

See discussion on that topic here:
 http://patchwork.ozlabs.org/patch/502558/

See similar patches for other architectures/platforms  here:
 [1] http://git.denx.de/?p=u-boot.git;a=commit;h=ff560a13056a565a4e9ce1761bd04276a3cace88
 [2] http://git.denx.de/?p=u-boot.git;a=commit;h=589907e2c187ec69b351c38ccda36730d25ab5d6

And while at it add missing shell prompt to axs103.

Cc: Tom Rini <trini@konsulko.com>
Cc: Joe Hershberger <joe.hershberger@gmail.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>