rules.mak: New logical functions for handling y/n values

Add new logical functions for handling y/n values like those we
use in CONFIG_FOO variables:
 lnot : logical NOT
 land : logical AND
 lor : logical OR
 lxor : logical XOR
 leqv : logical equality, inverse of lxor
 lif : like Make's $(if) but with an eq-like test

Based on an idea by Ákos Kovács <akoskovacs@gmx.com>.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

rules.mak

@@ -89,6 +89,23 @@ find-in-path = $(if $(find-string /, $1), \
         $(wildcard $1), \
         $(wildcard $(patsubst %, %/$1, $(subst :, ,$(PATH)))))
 
+# Logical functions (for operating on y/n values like CONFIG_FOO vars)
+# Inputs to these must be either "y" (true) or "n" or "" (both false)
+# Output is always either "y" or "n".
+# Usage: $(call land,$(CONFIG_FOO),$(CONFIG_BAR))
+# Logical NOT
+lnot = $(if $(subst n,,$1),n,y)
+# Logical AND
+land = $(if $(findstring yy,$1$2),y,n)
+# Logical OR
+lor = $(if $(findstring y,$1$2),y,n)
+# Logical XOR (note that this is the inverse of leqv)
+lxor = $(if $(filter $(call lnot,$1),$(call lnot,$2)),n,y)
+# Logical equivalence (note that leqv "","n" is true)
+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,$3)
+
 # Generate files with tracetool
 TRACETOOL=$(PYTHON) $(SRC_PATH)/scripts/tracetool.py