external-sourcery-toolchain: kill more linux-libc-headers headers

This avoids sysroot file conflict warnings when using a separate
linux-libc-headers recipe.

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
external-sourcery-toolchain: don't sed the mtd header if it doesn't exist

Signed-off-by: Christopher Larson <chris_larson@mentor.com>