xen: Add MISRA support to cppcheck make rule

xen: Add MISRA support to cppcheck make rule

cppcheck MISRA addon can be used to check for non compliance to some of
the MISRA standard rules.

Add a CPPCHECK_MISRA variable that can be set to "y" using make command
line to generate a cppcheck report including cppcheck misra checks.

When MISRA checking is enabled, a file with a text description suitable
for cppcheck misra addon is generated out of Xen documentation file
which lists the rules followed by Xen (docs/misra/rules.rst).

By default MISRA checking is turned off.

While adding cppcheck-misra files to gitignore, also fix the missing /
for htmlreport gitignore

Signed-off-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Michal Orzel <michal.orzel@arm.com>
Tested-by: Michal Orzel <michal.orzel@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>