util/abuild: Regenerate xcompile on every abuild run

Currently, running abuild in a fresh checkout without having built the
toolchain results in the following confusing behaviour:
1. Run abuild. It fails due to the missing coreboot toolchain, and the
   error message suggests running `make crossgcc`.
2. Run `make crossgcc`. It succeeds.
3. Re-run abuild. It still fails due to a missing coreboot toolchain.

This happens because the first abuild run generates an xcompile file
which uses the system toolchain. The second abuild run doesn't
regenerate the xcompile, so it still fails due to the non-coreboot
toolchain.

To avoid this confusing behaviour, regenerate the xcompile file every
time abuild is run.

BUG=None
TEST=Perform the steps above in a clean checkout. The second abuild run
now succeeds.

Signed-off-by: Reka Norman <rekanorman@google.com>
Change-Id: I78a7702c45cecbfe8460ec55df03741e5ced94b3
Reviewed-on: https://review.coreboot.org/c/coreboot/+/57651
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

util/abuild/abuild

@@ -788,6 +788,7 @@ fi
 
 # Generate a single xcompile for all boards
 export xcompile="${TARGET}/xcompile"
+rm -f "${xcompile}"
 $MAKE -C"${ROOT}" obj="$TARGET/temp" objutil="$TARGET/sharedutils" UPDATED_SUBMODULES=1 "${xcompile}" || exit 1
 
 customizing=$(echo "$customizing" | cut -c3-)