kconfig: Use HOST_OS environment variable in Makefile

Since the root Makefile already provides a HOST_OS
environment variable that is exported by using uname,
make use of it in the Kconfig Makefile.

Change-Id: I13655a5295bbcd9f2fdfa8b6309634c1ab143f70
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
diff --git a/scripts/kconfig/Makefile b/scripts/kconfig/Makefile
index 8873447..79bd3e3 100644
--- a/scripts/kconfig/Makefile
+++ b/scripts/kconfig/Makefile
@@ -11,8 +11,7 @@
 Kconfig := Kconfig
 endif
 
-UNAME := $(shell uname)
-ifeq (MINGW, $(findstring MINGW, $(UNAME)))
+ifeq ($(HOST_OS),MINGW)
 HOST_LOADLIBES ?= -lregex
 export HOST_LOADLIBES
 endif