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>
diff --git a/configs/axs103_defconfig b/configs/axs103_defconfig
index 8315b61..b3dbb8d 100644
--- a/configs/axs103_defconfig
+++ b/configs/axs103_defconfig
@@ -1,8 +1,8 @@
 CONFIG_ARC=y
 CONFIG_ISA_ARCV2=y
-CONFIG_TARGET_AXS101=y
 CONFIG_SYS_CLK_FREQ=50000000
 CONFIG_SYS_TEXT_BASE=0x81000000
+CONFIG_SYS_PROMPT="AXS# "
 # CONFIG_CMD_IMLS is not set
 # CONFIG_CMD_FLASH is not set
 # CONFIG_CMD_SETEXPR is not set