samples/bpf: Add makefile.target for separate CC target build

The Makefile.target is added only and will be used in
sample/bpf/Makefile later in order to switch cross-compiling to CC
from HOSTCC environment.

The HOSTCC is supposed to build binaries and tools running on the host
afterwards, in order to simplify build or so, like "fixdep" or else.
In case of cross compiling "fixdep" is executed on host when the rest
samples should run on target arch. In order to build binaries for
target arch with CC and tools running on host with HOSTCC, lets add
Makefile.target for simplicity, having definition and routines similar
to ones, used in script/Makefile.host. This allows later add
cross-compilation to samples/bpf with minimum changes.

The tprog stands for target programs built with CC.

Makefile.target contains only stuff needed for samples/bpf, potentially
can be reused later and now needed only for unblocking tricky
samples/bpf cross compilation.
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191011002808.28206-8-ivan.khoronzhuk@linaro.org

samples/bpf/Makefile.target

+# SPDX-License-Identifier: GPL-2.0
+# ==========================================================================
+# Building binaries on the host system
+# Binaries are not used during the compilation of the kernel, and intended
+# to be build for target board, target board can be host of course. Added to
+# build binaries to run not on host system.
+#
+# Sample syntax
+# tprogs-y := xsk_example
+# Will compile xsk_example.c and create an executable named xsk_example
+#
+# tprogs-y    := xdpsock
+# xdpsock-objs := xdpsock_1.o xdpsock_2.o
+# Will compile xdpsock_1.c and xdpsock_2.c, and then link the executable
+# xdpsock, based on xdpsock_1.o and xdpsock_2.o
+#
+# Derived from scripts/Makefile.host
+#
+__tprogs := $(sort $(tprogs-y))
+
+# C code
+# Executables compiled from a single .c file
+tprog-csingle	:= $(foreach m,$(__tprogs), \
+			$(if $($(m)-objs),,$(m)))
+
+# C executables linked based on several .o files
+tprog-cmulti	:= $(foreach m,$(__tprogs),\
+			$(if $($(m)-objs),$(m)))
+
+# Object (.o) files compiled from .c files
+tprog-cobjs	:= $(sort $(foreach m,$(__tprogs),$($(m)-objs)))
+
+tprog-csingle	:= $(addprefix $(obj)/,$(tprog-csingle))
+tprog-cmulti	:= $(addprefix $(obj)/,$(tprog-cmulti))
+tprog-cobjs	:= $(addprefix $(obj)/,$(tprog-cobjs))
+
+#####
+# Handle options to gcc. Support building with separate output directory
+
+_tprogc_flags   = $(TPROGS_CFLAGS) \
+                 $(TPROGCFLAGS_$(basetarget).o)
+
+# $(objtree)/$(obj) for including generated headers from checkin source files
+ifeq ($(KBUILD_EXTMOD),)
+ifdef building_out_of_srctree
+_tprogc_flags   += -I $(objtree)/$(obj)
+endif
+endif
+
+tprogc_flags    = -Wp,-MD,$(depfile) $(_tprogc_flags)
+
+# Create executable from a single .c file
+# tprog-csingle -> Executable
+quiet_cmd_tprog-csingle 	= CC  $@
+      cmd_tprog-csingle	= $(CC) $(tprogc_flags) $(TPROGS_LDFLAGS) -o $@ $< \
+		$(TPROGS_LDLIBS) $(TPROGLDLIBS_$(@F))
+$(tprog-csingle): $(obj)/%: $(src)/%.c FORCE
+	$(call if_changed_dep,tprog-csingle)
+
+# Link an executable based on list of .o files, all plain c
+# tprog-cmulti -> executable
+quiet_cmd_tprog-cmulti	= LD  $@
+      cmd_tprog-cmulti	= $(CC) $(tprogc_flags) $(TPROGS_LDFLAGS) -o $@ \
+			  $(addprefix $(obj)/,$($(@F)-objs)) \
+			  $(TPROGS_LDLIBS) $(TPROGLDLIBS_$(@F))
+$(tprog-cmulti): $(tprog-cobjs) FORCE
+	$(call if_changed,tprog-cmulti)
+$(call multi_depend, $(tprog-cmulti), , -objs)
+
+# Create .o file from a single .c file
+# tprog-cobjs -> .o
+quiet_cmd_tprog-cobjs	= CC  $@
+      cmd_tprog-cobjs	= $(CC) $(tprogc_flags) -c -o $@ $<
+$(tprog-cobjs): $(obj)/%.o: $(src)/%.c FORCE
+	$(call if_changed_dep,tprog-cobjs)