macros: Don't use Option in singleton!()

As detailed in rust-embedded/cortex-m#364, niche optimization of the
`Option` used in the `singleton!()` macro can lead to the initial value
of the static to contain non-zero bits.  This in turn leads to the whole
static being moved from `.bss` to `.data` which means it eats up flash
space for no reason.  Especially if the singleton stores a particularly
large type, this can be quite problematic.

Prevent this by using an explicit boolean flag instead of the `Option`
type.  This is not quite as nice but at least there is no chance for the
`singleton!()` to end up in `.data`...

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