Here is a missing register def for STM32L4 after support for dual RX. Thanks to Paul for porting the CAN improvements to both STM32 targets.

stm32_can

Shifting 1 by 31 is undefined behaviour because 1 is signed.
We should probably use 1ul instead of 1 everywhere else,
but for now this silences a compiler warning.

Add logic to support custom board directories that include a Kconfig file.  During the context phase of the build, any Kconfig file in the custom board directory is copied into configs/dummy, replacing the existing Kconfig file with the target Kconfig file.

 - Bugfix: stdfilters didn't work because the filter was never enabled (wrong number of bits to shift)
 - Bugfix: Filters were never used because the configuration register cannot be written without using the initialization mode

Both bugs are fixed by this patch. Filtering has been tested with both standard and extended identifiers and is now working properly.

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STMF7xxx RTC

 Remove proxy #defines
 Ensure the LSE(ON) etal are set and remembered in
  a) A cold start (RTC_MAGIC invalid) of the RTC
  b) A warm start (RTC_MAGIC valid) of the RTC but a clock change.

The change was needed because in bench testing a merge of the latest's STM32  53ec3ca1 (and friends) it became apparent that the
Sequence of operation is wrong in the reset of the Backup Domain in the RCC code.  PWR is required before the Backup Domain
can be futzed with. !!!This Code should be tested on STM32 and if needed rippled to the STM32 families

Remove configs/stm32f746g-disco/netnsh.  It does not build.  Probably a corrupted, hand-editted defconfig file.

STM32 CAN: Clone missing stm32_enterinitmode() and _exitinitmode() from STM32L4.  Don't know if this is write but is needed to compile.

efm32, lcp43, stm32, stm32l4: disable interrupts with NVIC_IRQ_CLEAR

This change fixes a build problem that only occurs when reconfiguring from Linux to Windows or vice-versa.  It is a problem that was present but not usually experienced until two things happened:  (1) The pre_config target was added to run before the menconfig operation and (2) the context target was added before the pre_config target in order to set up the correct symbolic links (in the apps/platform directory) needed by the pre_config target.

But then now if you start with a Linux system and run 'make menuconfig' to switch to Linux, the context target will execute first and set up POSIX style symbolic links before doing the menuconfig.  Then after the menuconfig, the make will fail on Windows if you are using a Windows native toolchain because that native toolchain cannot follow the Cygwin-style symbolic links.

The first here is to also execute the clean_context AFTER executing menuconfig.  A lot more happens now:  It used to be that doing 'make menuconfig' only did the menuconfig operation.  No it does context, pre_config, menuconfig, clean_context.  Not nearly as snappy as it used to be.

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