Add a rule to test if a string starts with a substring.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 rules.mak | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/rules.mak b/rules.mak
index 694865b63e..e39bee93d5 100644
--- a/rules.mak
+++ b/rules.mak
@@ -191,6 +191,13 @@ ne = $(if $(subst $2,,$1)$(subst $1,,$2),y,n)
 isempty = $(if $1,n,y)
 notempty = $(if $1,y,n)
 
+# startwith
+# Usage: $(call startwith, startstr, fullstr)
+#
+# This macro returns a string (TRUE) when @fullstr starts with
+# @startstr, else returns the empty string (FALSE)
+startwith = $(findstring :$(strip $1),:$(strip $2))
+
 # Generate files with tracetool
 TRACETOOL=$(PYTHON) $(SRC_PATH)/scripts/tracetool.py
 
-- 
2.21.3