mk: add global-cppflags-y

Add global-cppflags-y as a new sub.mk variable. The content of the
variable is added to the C preprocessor flags passed to all files
compiled in the current submodule, $(sm), similarly to how the content
of global-incdirs-y is passed on.

This flag is intended for libraries that in addition to exporting an
include path also need certain defines set.

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Etienne Carriere <etienne.carriere@foss.st.com>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>

mk/subdir.mk

@@ -140,6 +140,7 @@ endif
 
 include $1/sub.mk
 sub-subdirs := $$(addprefix $1/,$$(subdirs-y)) $$(subdirs_ext-y)
+cppflags$(sm) := $$(cppflags$(sm)) $$(global-cppflags-y)
 incdirs$(sm) := $(incdirs$(sm)) $$(addprefix $1/,$$(global-incdirs-y))
 thissubdir-incdirs := $(out-dir)/$(base-prefix)$1 $$(addprefix $1/,$$(incdirs-y)) $$(incdirs_ext-y)
 ifneq ($$(libname),)
@@ -168,6 +169,7 @@ aflags-remove-y :=
 subdirs-y :=
 subdirs_ext-y :=
 global-incdirs-y :=
+global-cppflags-y :=
 incdirs-lib-y :=
 incdirs-y :=
 incdirs_ext-y :=