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>

arch/arc/Kconfig

@@ -129,7 +129,7 @@ config ARC_CACHE_LINE_SHIFT
 
 choice
 	prompt "Target select"
-	optional
+	default TARGET_AXS101
 
 config TARGET_TB100
 	bool "Support tb100"

configs/axs101_defconfig

@@ -1,13 +1,12 @@
 CONFIG_ARC=y
 CONFIG_SYS_DCACHE_OFF=y
 CONFIG_ARC_CACHE_LINE_SHIFT=5
-CONFIG_TARGET_AXS101=y
 CONFIG_SYS_CLK_FREQ=750000000
 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
 CONFIG_NETDEVICES=y
 CONFIG_ETH_DESIGNWARE=y
 CONFIG_USE_PRIVATE_LIBGCC=y
-CONFIG_SYS_PROMPT="AXS# "

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