ARC: rework setting of ARC CPU specific compiler options

It's a very rare if at all existing occasion when ARC CPU template
is used as is w/o any changes - in the end it's a beauty and competitive
advantage of ARC cores to be tailored for a particular use-case - and
so it doesn't make a lot of sense to offer template-based "-mcpu" selection.
Given for each and every platform we end-up adding quite a few more flags
it's logical to move "-mcpu" selection to platform's definition as well
which we exactly do here.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
diff --git a/board/synopsys/nsim/Kconfig b/board/synopsys/nsim/Kconfig
index 2228703..b6966b2 100644
--- a/board/synopsys/nsim/Kconfig
+++ b/board/synopsys/nsim/Kconfig
@@ -9,4 +9,13 @@
 config SYS_CONFIG_NAME
 	default "nsim"
 
+config NSIM_BOARD_CPPFLAGS
+	string "board arc-specific compiler options"
+	help
+	  For nSIM we allow to set custom arc-specific compiler options
+	  (like -mcpu=) instead of hardcoding them in its makefile as nSIM
+	  target is used for representing targets without fixed CPU version
+	  like FPGA-based boards and software simulators.
+	  This variable takes space separated compiler options list.
+
 endif