Use sparse files for the generated disk images on all OSes

Rather than just on macOS. This can save quite a bit of space depending
on the amount of free space on the root filesystem of the disk image.

mk/config.mk

@@ -41,7 +41,6 @@ ifeq ($(UNAME),Darwin)
 	VB_AUDIO=coreaudio
 	VBM=/Applications/VirtualBox.app/Contents/MacOS/VBoxManage
 	HOST_TARGET ?= $(HOST_ARCH)-apple-darwin
-	ALLOC_FILE=truncate -s "$(FILESYSTEM_SIZE)m"
 else ifeq ($(UNAME),FreeBSD)
 	FUMOUNT=sudo umount
 	export NPROC=sysctl -n hw.ncpu
@@ -49,7 +48,6 @@ else ifeq ($(UNAME),FreeBSD)
 	VB_AUDIO=pulse # To check, will probaly be OSS on most setups
 	VBM=VBoxManage
 	HOST_TARGET ?= $(HOST_ARCH)-unknown-freebsd
-	ALLOC_FILE=fallocate --posix --length "$(FILESYSTEM_SIZE)MiB"
 else
 	# Detect which version of the fusermount binary is available.
 	ifneq (, $(shell which fusermount3))
@@ -63,7 +61,6 @@ else
 	VB_AUDIO=pulse
 	VBM=VBoxManage
 	HOST_TARGET ?= $(HOST_ARCH)-unknown-linux-gnu
-	ALLOC_FILE=fallocate --posix --length "$(FILESYSTEM_SIZE)MiB"
 endif
 
 ifneq ($(UNAME),Linux)