Build the Linux kernel independently from Buildroot

This will allow for other types of root filesystems that don't rely on Buildroot
to be added and used in the future.

Propagate --verbose on all build scripts to see full GCC commands.

build-all: allow for neat subsets

also 9p share rootfs_overlay. TODO document.

packages/kernel_modules/Makefile

@@ -1,10 +1,7 @@
-obj-m += $(addsuffix .o, $(notdir $(basename $(filter-out %.mod.c, $(wildcard $(BR2_EXTERNAL_KERNEL_MODULES_PATH)/*.c)))))
+obj-m += $(OBJECT_FILES)
 ccflags-y := -DDEBUG -g -std=gnu99 -Werror -Wno-declaration-after-statement -Wframe-larger-than=1000000000
 
-.PHONY: all clean
+.PHONY: all
 
 all:
-	$(MAKE) -C '/lib/modules/$(shell uname -r)/build' M='$(PWD)' modules
-
-clean:
-	$(MAKE) -C '$(LINUX_DIR)' M='$(PWD)' clean
+	$(MAKE) -C '$(LINUX_DIR)' M='$(M)'

packages/kernel_modules/make-host.sh

@@ -1,24 +0,0 @@
-#!/usr/bin/env bash
-
-# We can almost do everything from the Makefile itself by using default values for
-#
-#    LINUX_DIR ?= "/lib/modules/$(uname -r)/build"
-#    BR2_EXTERNAL_KERNEL_MODULES_PATH="$(pwd)"
-#
-# The problem with that is that if you define those variables in your environment,
-# the build breaks, so this is more portable.
-#
-# Trying to add `-i` to overcome incompatible modules will fail,
-# because any build failure prevents the generation of all `.mod.c` files.
-
-j="$(($(nproc) - 2))"
-while getopts j: OPT; do
-  case "$OPT" in
-    'j')
-      j="$OPTARG"
-    ;;
-  esac
-done
-shift $(($OPTIND - 1))
-make -j "$j" KERNEL_MODULES_PATH="$(pwd)" LINUX_DIR="/lib/modules/$(uname -r)/build" "$@"
-make -C user/ -j "$j" "$@"

packages/kernel_modules/user/Makefile

@@ -1,12 +1,13 @@
-.PHONY: all clean
+.PHONY: all clean mkdir
 
 CFLAGS_EXTRA = -fopenmp -std=c99
 CXXFLAGS_EXTRA = -std=c++17
 CCFLAGS_EXTRA = -Wall -Werror -Wextra
 IN_EXT_C = .c
 IN_EXT_CXX = .cpp
-LIBS =
+LIBS = -lm
 OUT_EXT = .out
+OUT_DIR = .
 
 OUTS := $(foreach IN_EXT,$(IN_EXT_C) $(IN_EXT_CXX),$(addsuffix $(OUT_EXT), $(basename $(wildcard *$(IN_EXT)))))
 ifeq ($(BR2_PACKAGE_EIGEN),y)
@@ -30,14 +31,18 @@ ifeq ($(BR2_PACKAGE_OPENBLAS),y)
 else
 	OUTS := $(filter-out openblas$(OUT_EXT),$(OUTS))
 endif
+OUTS := $(addprefix $(OUT_DIR)/,$(OUTS))
 
-all: $(OUTS)
+all: mkdir $(OUTS)
 
-%$(OUT_EXT): %$(IN_EXT_C)
+$(OUT_DIR)/%$(OUT_EXT): %$(IN_EXT_C)
 	$(CC) $(CFLAGS) $(CFLAGS_EXTRA) -o '$@' '$<' $(LIBS)
 
-%$(OUT_EXT): %$(IN_EXT_CXX)
+$(OUT_DIR)/%$(OUT_EXT): %$(IN_EXT_CXX)
 	$(CXX) $(CXXFLAGS) $(CXXFLAGS_EXTRA) -o '$@' '$<' $(LIBS)
 
 clean:
 	rm -f *'$(OUT_EXT)'
+
+mkdir:
+	mkdir -p '$(OUT_DIR)'