[make help] mention INITCPIO switch (#137)

This commit is an expansion to :
87b816345b

Since a new `make` parameter has been introduced, it should be mentioned
in the output of the `help` target.

Also, the table is being expanded by a new column describing the type of
data that each parameter may be assigned, like paths or boolean values.

Makefile

@@ -1,6 +1,8 @@
 PKGNAME ?= grub-btrfs
 PREFIX ?= /usr
 
+INITCPIO ?= false
+
 SHARE_DIR = $(DESTDIR)$(PREFIX)/share
 LIB_DIR = $(DESTDIR)$(PREFIX)/lib
 
@@ -13,7 +15,7 @@ install:
 	@install -Dm644 -t "$(LIB_DIR)/systemd/system/" grub-btrfs.path
 	@install -Dm644 -t "$(SHARE_DIR)/licenses/$(PKGNAME)/" LICENSE
 	@# Arch Linux like distros only :
-	@if command -V mkinitcpio >/dev/null 2>&1; then \
+	@if test "$(INITCPIO)" = true; then \
 		install -Dm644 "initramfs/Arch Linux/overlay_snap_ro-install" "$(LIB_DIR)/initcpio/install/grub-btrfs-overlayfs"; \
 		install -Dm644 "initramfs/Arch Linux/overlay_snap_ro-hook" "$(LIB_DIR)/initcpio/hooks/grub-btrfs-overlayfs"; \
 	 fi
@@ -29,8 +31,8 @@ uninstall:
 	@rm -f "$(LIB_DIR)/systemd/system/grub-btrfs.path"
 	@rm -f "$(LIB_DIR)/initcpio/install/grub-btrfs-overlayfs"
 	@rm -f "$(LIB_DIR)/initcpio/hooks/grub-btrfs-overlayfs"
-	@# Arch Linux unlike distros only :
-	@if ! command -V mkinitcpio >/dev/null 2>&1; then \
+	@# Arch Linux UNlike distros only :
+	@if test "$(INITCPIO)" != true && test -d "$(LIB_DIR)/initcpio"; then \
 		rmdir --ignore-fail-on-non-empty "$(LIB_DIR)/initcpio/install" || :; \
 		rmdir --ignore-fail-on-non-empty "$(LIB_DIR)/initcpio/hooks" || :; \
 		rmdir --ignore-fail-on-non-empty "$(LIB_DIR)/initcpio" || :; \
@@ -50,11 +52,12 @@ help:
 	@echo "           uninstall"
 	@echo "           help"
 	@echo
-	@echo "  parameters | description                    | defaults"
-	@echo "  -----------+--------------------------------+----------------------------"
-	@echo "  DESTDIR    | install destination            | <unset>"
-	@echo "  PREFIX     | system tree prefix             | '/usr'"
-	@echo "  SHARE_DIR  | shared data location           | '\$$(DESTDIR)\$$(PREFIX)/share'"
-	@echo "  LIB_DIR    | system libraries location      | '\$$(DESTDIR)\$$(PREFIX)/lib'"
-	@echo "  PKGNAME    | name of the ditributed package | 'grub-btrfs'"
+	@echo "  parameter | type | description                    | defaults"
+	@echo "  ----------+------+--------------------------------+----------------------------"
+	@echo "  DESTDIR   | path | install destination            | <unset>"
+	@echo "  PREFIX    | path | system tree prefix             | '/usr'"
+	@echo "  SHARE_DIR | path | shared data location           | '\$$(DESTDIR)\$$(PREFIX)/share'"
+	@echo "  LIB_DIR   | path | system libraries location      | '\$$(DESTDIR)\$$(PREFIX)/lib'"
+	@echo "  PKGNAME   | name | name of the ditributed package | 'grub-btrfs'"
+	@echo "  INITCPIO  | bool | include mkinitcpio hook        | false"
 	@echo