include/version.h: workaround sysroot inc order

On some systems the host system or even the cross sysroot can
contain a version.h.  This leads to the wrong file being picked
up and a PLAIN_VERSION undefined error.

This workaround symlinks the version.h into the tool folder to
allow reordering of search folders.

Fixes
http://autobuild.buildroot.net/results/770/7702d5df36a6532aafdbe6e9e62709bbfa058b54/build-end.log
http://autobuild.buildroot.net/results/e34/e3401027d2fb3ce565ca9e2456a427afd3610a87/build-end.log
... additional can be found with these queries ...
http://autobuild.buildroot.net/?reason=uboot-tools-2018.03
http://autobuild.buildroot.net/?reason=uboot-tools-2018.05

Upstream: pending

Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>

tools/env/Makefile

@@ -9,7 +9,8 @@
 override HOSTCC = $(CC)
 
 # Compile for a hosted environment on the target
-HOST_EXTRACFLAGS  = $(patsubst -I%,-idirafter%, $(filter -I%, $(UBOOTINCLUDE))) \
+HOST_EXTRACFLAGS  = -I$(srctree)/tools \
+		$(patsubst -I%,-idirafter%, $(filter -I%, $(UBOOTINCLUDE))) \
 		-idirafter $(srctree)/tools/env \
 		-DUSE_HOSTCC \
 		-DTEXT_BASE=$(TEXT_BASE)

tools/version.h

@@ -0,0 +1 @@
+../include/version.h