targets/qemu.mk: use exfat for USB image to support >4GB ISOs

VFAT (FAT32) has a 4GB file size limit which breaks large ISOs like:
- ubuntu-25.10-desktop-amd64.iso (5.7GB)
- Qubes-R4.3.0-x86_64.iso (8.2GB)

exfat has no file size limit and is supported by most Linux distributions
and the heads initrd (CONFIG_EXFAT_FS=y in all configs).

Signed-off-by: Thierry Laurion <insurgo@riseup.net>

Author: tlaurion

targets/qemu.mk

@@ -45,12 +45,8 @@ $(MEMORY_SIZE_FILE):
 	@echo "$(QEMU_MEMORY_SIZE)" >"$(MEMORY_SIZE_FILE)"
 USB_FD_IMG=$(build)/$(BOARD)/usb_fd.raw
 $(USB_FD_IMG):
-	dd if=/dev/zero bs=1M of="$(USB_FD_IMG)" bs=1M count=256 >/dev/null 2>&1
-	# Debian obnoxiously does not include /usr/sbin in PATH for non-root, even
-	# though it is meaningful to use mkfs.vfat (etc.) as non-root
-	MKFS_VFAT=mkfs.vfat; \
-	[ -x /usr/sbin/mkfs.vfat ] && MKFS_VFAT=/usr/sbin/mkfs.vfat; \
-	"$$MKFS_VFAT" "$(USB_FD_IMG)"
+	dd if=/dev/zero bs=1M of="$(USB_FD_IMG)" bs=1G count=64 >/dev/null 2>&1
+	mkfs.exfat "$(USB_FD_IMG)"
 # Pass INSTALL_IMG=<path_to_img.iso> to attach an installer as a USB flash drive instead
 # of the temporary flash drive for exporting GPG keys.
 ifneq "$(INSTALL_IMG)" ""