Do not include headers inside __BEGIN_DECLS ... __END_DECLS blocks.

We don't have C++ unsafe headers (anymore).

I added a test to fix_headers.sh that checks if certain "unsafe"
headers are ONLY included inside a __BEGIN_DECLS ... __END_DECLS
(because after all, they are unsafe), as well as checking that
no other header files are included inside such a block. The rationale
of the latter is that if a file is a C header and it declares
function prototypes (otherwise it doesn't matter) and is sometimes
included outside a __BEGIN_DECLS ... __END_DECLS block (from a C++
source file) then it has to be C++ safe and doesn't ever to be
included from inside such a block; while if a file is a C++ header
then obviously it should never be included from such a block.

fix_headers.sh subsequently found several safe headers to be
included from inside such a block, and those that were (apparently
in the past) unsafe were included only sometimes inside such a
block and often outside it. I had a look at those files and saw
that at least an attempt has been made to make them C++ safe,
but especially because they already are included OFTEN outside
a __BEGIN_DECLS ... __END_DECLS (from C++ source files) the
best decision seems to treat them as safe.

This is not risky: .c files that define such functions still
generate C-linkage for their functions. If a C++ unsafe C header
is included outside a __BEGIN_DECLS ... __END_DECLS block then
the only possible result would be an undefined reference to
a function with C++-linkage that will not exist. Aka, when
something links after this commit, then the commit was correct.
I did build all targets and they all linked.

src/drivers/boards/crazyflie/board_config.h

@@ -47,9 +47,6 @@
 #include <nuttx/compiler.h>
 #include <stdint.h>
 
-__BEGIN_DECLS
-
-/* these headers are not C++ safe */
 #include <stm32.h>
 #include <arch/board/board.h>
 
@@ -168,8 +165,7 @@ __BEGIN_DECLS
 
 #define BOARD_FMU_GPIO_TAB { {0, 0, 0}, }
 
-
-
+__BEGIN_DECLS
 
 /****************************************************************************************************
  * Public Types
@@ -219,9 +215,6 @@ extern void board_peripheral_reset(int ms);
 int nsh_archinitialize(void);
 #endif
 
-
-
-
 /****************************************************************************
  * Name: board_i2c_initialize
  *
@@ -232,7 +225,6 @@ int nsh_archinitialize(void);
 
 int board_i2c_initialize(void);
 
-
 #endif /* __ASSEMBLY__ */
 
 __END_DECLS