Revert "kernel: Use CPATH for including host headers"

This reverts commit 2baeb4f.

CPATH is used for both host and target. Meaning that system headers
will be used for target compilation. It is also a higher priority than
isystem, which is used for the compiler include paths, making this
override the compiler headers.

Change-Id: I4d836698f25d2d97d917d323a22ee359b937fcb4

config/BoardConfigKernel.mk

@@ -169,7 +169,7 @@ ifneq ($(KERNEL_NO_GCC), true)
         endif
     endif
 
-    KERNEL_MAKE_FLAGS += CPATH="/usr/include:/usr/include/x86_64-linux-gnu" HOSTLDFLAGS="-L/usr/lib/x86_64-linux-gnu -L/usr/lib64 -fuse-ld=lld"
+    KERNEL_MAKE_FLAGS += HOSTCFLAGS="-I/usr/include -I/usr/include/x86_64-linux-gnu" HOSTLDFLAGS="-L/usr/lib/x86_64-linux-gnu -L/usr/lib64 -fuse-ld=lld"
 
     ifeq ($(KERNEL_ARCH),arm64)
         # Add 32-bit GCC to PATH so that arm-linux-androidkernel-as is available for CONFIG_COMPAT_VDSO