[build] Don't assume the existence of "seq"

The "seq" command is GNU-specific; a BSD userland will not have it.
Use POSIX-conforming "awk" instead.

Reported-by: Joshua Oreman <oremanj@rwcr.net>
Suggested-by: Stefan Hajnoczi <stefanha@gmail.com>

src/Makefile.housekeeping

 echo :
 	@$(ECHO) "Using \"$(ECHO_E)\" for \"echo -e\""
 
+###############################################################################
+#
+# Generate a usable "seq" substitute
+#
+define seq
+	$(shell awk 'BEGIN { for ( i = $(1) ; i <= $(2) ; i++ ) print i }')
+endef
+
 ###############################################################################
 #
 # Determine host OS
 VERYCLEANUP	+= $(EMBEDDED_LIST)
 
 EMBEDDED_FILES	:= $(subst $(COMMA), ,$(EMBEDDED_IMAGE))
-EMBED_ALL	:= $(foreach i,$(shell seq 1 $(words $(EMBEDDED_FILES))),\
+EMBED_ALL	:= $(foreach i,$(call seq,1,$(words $(EMBEDDED_FILES))),\
 		     EMBED ( $(i), \"$(word $(i), $(EMBEDDED_FILES))\",\
 			     \"$(notdir $(word $(i),$(EMBEDDED_FILES)))\" ))