Revert "newlib-nano: add include directory"

This reverts commit 88aaa623.

makefiles/libc/newlib.mk

@@ -64,8 +64,7 @@ ifeq ($(TOOLCHAIN),llvm)
 endif
 
 ifeq (1,$(USE_NEWLIB_NANO))
-  # newlib-nano include directory is called either newlib-nano or nano. Use the one we find first.
-  NEWLIB_NANO_INCLUDE_DIR ?= $(firstword $(wildcard $(addprefix $(NEWLIB_INCLUDE_DIR)/, newlib-nano nano)))
+  NEWLIB_NANO_INCLUDE_DIR ?= $(NEWLIB_INCLUDE_DIR)/newlib-nano
   # newlib-nano overrides newlib.h and its include dir should therefore go before
   # the regular system include dirs.
   INCLUDES := -isystem $(NEWLIB_NANO_INCLUDE_DIR) $(INCLUDES)