build: provide xxd -i with scripts/xxdi.pl

Dependency on xxd was added in commit c4dd2441e7 ("tools: add xxd
(from vim)") as U-Boot requires xxd to create the default environment
from an external file.

Later in commit 2b94aac7a1 ("tools: xxd: use more convenient source
tarball"), xxd from another source was used instead, but that source is
currently unavailable, so let's fix it by using simple xxdi.pl Perl
script instead.

Fixes: #10555
Signed-off-by: Petr Štetiar <ynezz@true.cz>
(cherry picked from commit eae2fb8027)

include/prereq-build.mk

@@ -192,7 +192,10 @@ $(STAGING_DIR_HOST)/bin/mkhash: $(SCRIPT_DIR)/mkhash.c
 	mkdir -p $(dir $@)
 	$(CC) -O2 -I$(TOPDIR)/tools/include -o $@ $<
 
-prereq: $(STAGING_DIR_HOST)/bin/mkhash
+$(STAGING_DIR_HOST)/bin/xxd: $(SCRIPT_DIR)/xxdi.pl
+	$(LN) $< $@
+
+prereq: $(STAGING_DIR_HOST)/bin/mkhash $(STAGING_DIR_HOST)/bin/xxd
 
 # Install ldconfig stub
 $(eval $(call TestHostCommand,ldconfig-stub,Failed to install stub, \