BUG: Fix mtools check errors reported by brchan on thinkpads forum

It seems that mtools behaves differently with different versions and
that some versions react badly to the FAT16 image used in the Lenovo
ISO updates.

    mcopy -o -i g5uj28us.iso@@71680 g5uj28us.iso.bat ::AUTOEXEC.BAT
    Total number of sectors (64195) not a multiple of sectors per track (32)!
    Add mtools_skip_check=1 to your .mtoolsrc file to skip this test

adding the skip_check environment variable simply ignores this error...

Makefile

@@ -116,7 +116,7 @@ $(DEPSDIR)/slice.insert.deps: Makefile
 
 # If we ever want a copy of the dosflash.exe, just get it from the iso image
 %.dosflash.exe.orig: %.iso.orig
-	mcopy -i $^@@$(FAT_OFFSET) ::FLASH/DOSFLASH.EXE $@
+	MTOOLS_SKIP_CHECK=1 mcopy -i $^@@$(FAT_OFFSET) ::FLASH/DOSFLASH.EXE $@
 
 mec-tools/Makefile:
 	git submodule update --init --remote
@@ -136,7 +136,7 @@ endef
 # $1 = FL2 filename
 # $2 = ISO image
 define rule_iso
-    $(2): $(1) $(2).bat ; ./slice.insert $(1).slice $(1) $(2) && mcopy -o -i $(2)@@$(FAT_OFFSET) $(2).bat ::AUTOEXEC.BAT
+    $(2): $(1) $(2).bat ; ./slice.insert $(1).slice $(1) $(2) && MTOOLS_SKIP_CHECK=1 mcopy -o -i $(2)@@$(FAT_OFFSET) $(2).bat ::AUTOEXEC.BAT
 endef
 
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