trees: unset CROSS_COMPILE per target

When building a coreboot image, if they enable the
x86 U-Boot payloads, sometimes what happens is you
have CROSS_COMPILE set, for i386-elf, but then it's
still set to that when later building 64-bit U-Boot,
which needs x86_64-elf.

We currently rely on hostcc to build U-Boot.

To mitigate this, unset CROSS_COMPILE in the main
loop of the trees script, for building project targets.

Signed-off-by: Leah Rowe <leah@libreboot.org>

script/trees

@@ -73,6 +73,7 @@ build_targets()
 	[ $# -gt 0 ] || targets="$(ls -1 "$configdir")" || $err "!o $configdir"
 
 	for x in $targets; do
+		unset CROSS_COMPILE
 		[ "$x" = "list" ] && x_ ls -1 "config/$project" && \
 		    listfile="" && break
 		target="$x"