rules.mak: New string testing functions

Add new string testing functions which return a y/n result:
 eq : are two strings equal (ignoring leading/trailing space)?
 ne : are two strings unequal?
 isempty : is a string empty?
 notempty : is a string non-empty?

Based on an idea by Ákos Kovács <[email protected]>.

Signed-off-by: Peter Maydell <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

rules.mak

@@ -106,6 +106,17 @@ leqv = $(if $(filter $(call lnot,$1),$(call lnot,$2)),y,n)
 # Logical if: like make's $(if) but with an leqv-like test
 lif = $(if $(subst n,,$1),$2,$3)
 
+# String testing functions: inputs to these can be any string;
+# the output is always either "y" or "n". Leading and trailing whitespace
+# is ignored when comparing strings.
+# String equality
+eq = $(if $(subst $2,,$1)$(subst $1,,$2),n,y)
+# String inequality
+ne = $(if $(subst $2,,$1)$(subst $1,,$2),y,n)
+# Emptiness/non-emptiness tests:
+isempty = $(if $1,n,y)
+notempty = $(if $1,y,n)
+
 # Generate files with tracetool
 TRACETOOL=$(PYTHON) $(SRC_PATH)/scripts/tracetool.py