arc: add selection of endianess in Kconfig

This change allows to keep board description clean and minimalistic.
This is especially helpful if one board may house different CPUs with
different features.

It is applicable to both FPGA-based boards or those that have CPUs
mounted on interchnagable daughter-boards.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
diff --git a/configs/arcangel4-be_defconfig b/configs/arcangel4-be_defconfig
index 0c3ab4e..5d2e126 100644
--- a/configs/arcangel4-be_defconfig
+++ b/configs/arcangel4-be_defconfig
@@ -1,3 +1,4 @@
 CONFIG_ARC=y
-CONFIG_TARGET_ARCANGEL4_BE=y
+CONFIG_TARGET_ARCANGEL4=y
 CONFIG_SYS_CLK_FREQ=70000000
+CONFIG_CPU_BIG_ENDIAN=y