kbuild: add `baseprereq'

On the same model as `basetarget', it represents the filename of first
prerequisite with directory and extension stripped.
Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>

scripts/Kbuild.include

@@ -20,6 +20,10 @@ depfile = $(subst $(comma),_,$(dot-target).d)
 # filename of target with directory and extension stripped
 basetarget = $(basename $(notdir $@))
 
+###
+# filename of first prerequisite with directory and extension stripped
+baseprereq = $(basename $(notdir $<))
+
 ###
 # Escape single quote for use in echo statements
 escsq = $(subst $(squote),'\$(squote)',$1)