modules/mqnic/: Fix and enhance Makefile

- Apply "Shared Makefile" style as found in [1]
- Especially but not only for cross compilation:
  - Accept commonly used variable KDIR to specify kernel build directory
  - Additionally accept KERNEL_SRC (instead of KDIR) as used by PetaLinux/Yocto
- Route all usual external module targets through to Kbuild
- Provide "install" target as shortcut for module installation
- Use $(MAKE) as recommended in [2]

[1] https://www.kernel.org/doc/html/latest/kbuild/modules.html, section 3.1
[2] https://www.gnu.org/software/make/manual/make.html#MAKE-Variable, section 5.7.1.

Signed-off-by: Joachim Foerster <joachim.foerster@missinglinkelectronics.com>

modules/mqnic/Makefile

@@ -1,3 +1,4 @@
+ifneq ($(KERNELRELEASE),)
 
 # object files to build
 obj-m += mqnic.o
@@ -16,9 +17,22 @@ mqnic-y += mqnic_cq.o
 mqnic-y += mqnic_eq.o
 mqnic-y += mqnic_ethtool.o
 
+else
+
+ifneq ($(KERNEL_SRC),)
+# alternatively to variable KDIR accept variable KERNEL_SRC as used in
+# PetaLinux/Yocto for example
+KDIR ?= $(KERNEL_SRC)
+endif
+
+KDIR ?= /lib/modules/$(shell uname -r)/build
+
 all:
-	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
+	$(MAKE) -C $(KDIR) M=$(shell pwd) modules
 
-clean:
-	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean
+help modules modules_install clean:
+	$(MAKE) -C $(KDIR) M=$(shell pwd) $@
 
+install: modules_install
+
+endif