Ensure Compatibility with BSD sha1sum

BSD and GNU implementations have slightly different behavior, the
former implementation is used in macOS, FreeBSD and similar systems.

While both implementations behave identically with the -c - argument
when the file exists, they differ when the file does not exist. This
commit adds an 'ls' command to address this difference, ensuring
consistent behavior across platforms.

mk/external.mk

@@ -68,7 +68,7 @@ define verify
                 | sort \
                 | $(SHA1SUM) \
                 | cut -f 1 -d ' ' > $(SHA1_FILE2) && cmp $(SHA1_FILE1) $(SHA1_FILE2))), \
-            ($(eval VERIFIER := echo "$(strip $(1))  $(strip $(2))" | $(SHA1SUM) -c)) \
+            ($(eval VERIFIER := (ls $(2) >/dev/null 2>&1 || echo FAILED) && echo "$(strip $(1))  $(strip $(2))" | $(SHA1SUM) -c -)) \
     ))
     $(eval _ := $(shell $(VERIFIER) 2>&1))
     $(eval _ := \