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>
diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
index be39cd1..d897278 100644
--- a/scripts/Kbuild.include
+++ b/scripts/Kbuild.include
@@ -21,6 +21,10 @@
 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)