Add a "reverse AUX hack" for ARM.

MIPS and x86 are almost completely switched over to uapi-only. ARM is still
currently old-only. A cleanup of the uapi headers over the weekend means
that the uapi-only <linux/kexec.h> is now in the right place, which will break
the ARM build unless the ARM build also looks in the uapi headers if it can't
find an old header.

Change-Id: Ie7a53ef2122b43cdef64b71d7b36a8381992a461
diff --git a/core/combo/TARGET_linux-arm.mk b/core/combo/TARGET_linux-arm.mk
index 6fbb778..64957bf 100644
--- a/core/combo/TARGET_linux-arm.mk
+++ b/core/combo/TARGET_linux-arm.mk
@@ -205,8 +205,9 @@
 else
     KERNEL_HEADERS_COMMON := $(libc_root)/kernel/common
     KERNEL_HEADERS_ARCH   := $(libc_root)/kernel/arch-$(TARGET_ARCH)
+    KERNEL_HEADERS_AUX    := $(libc_root)/kernel/uapi # for kexec.h
 endif
-KERNEL_HEADERS := $(KERNEL_HEADERS_COMMON) $(KERNEL_HEADERS_ARCH)
+KERNEL_HEADERS := $(KERNEL_HEADERS_COMMON) $(KERNEL_HEADERS_ARCH) $(KERNEL_HEADERS_AUX)
 
 TARGET_C_INCLUDES := \
 	$(libc_root)/arch-arm/include \