add a Cargo feature, cm7-r0p1, to fix a Cortex-M7 BASEPRI erratum

see japaric/cortex-m-rtfm#53 for background information

add a Cargo feature, cm7-r0p1, to fix a Cortex-M7 BASEPRI erratum

see japaric/cortex-m-rtfm#53 for background information

add a Cargo feature, cm7-r0p1, to fix a Cortex-M7 BASEPRI erratum

closes #53
alternative solution to #54
depends on rust-embedded/cortex-m#72

add a Cargo feature, cm7-r0p1, to fix a Cortex-M7 BASEPRI erratum

closes #53
alternative solution to #54
depends on rust-embedded/cortex-m#72

`exception!(HardFault, ..)` and `exception!(*, ..)` can now be omitted from
programs to pick up the default behavior of an infinite loop. Existing programs
that define these exception handlers will continue to work w/o any functional
change.

closes #72

85: [WIP] provide defaults for DefaultHandler and HardFault r=korken89 a=japaric

`exception!(HardFault, ..)` and `exception!(*, ..)` can now be omitted from
programs to pick up the default behavior of an infinite loop. Existing programs
that define these exception handlers will continue to work w/o any functional
change.

closes #72

--

Annoyances:

- The handlers can't be *just* an infinite loop because of rust-lang/rust#28728.
If we define the handlers as just `loop {}` they will become an abort (UDF)
instruction. And that would turn HardFault into infinite recursion. For that
reason I have made them into an infinite loop that does some side effect

- If you stick to the defaults then the symbol name of the default handler
changes from `DefaultHandler` (override) to `DefaultDefaultHandler` (default).
We can make these two names more similar but I think we can not prevent the
rename. Something similar happens with UserHardFault (which becomes
DefaultUserHardFault when not overridden).

cc @rust-embedded/cortex-m

Co-authored-by: Jorge Aparicio <jorge@japaric.io>