py/persistentcode: Add architecture flags check for RV32 platforms.

This commit introduces the MPY architecture flags checking code specific
for the RV32 target, currently checking for the only additional
extension that is supported by the runtime: Zba.

The warnings inside "mpy-cross" have also been removed since now there
is a way to reject incompatible MPY files at runtime.

Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

docs/reference/mpyfiles.rst

@@ -179,8 +179,19 @@ If bit #6 of the header's feature flags byte is set, then a vuint containing
 optional architecture-specific information will follow the header. The contents
 of this integer depends on which native architecture the file is meant for.
 
-See also ``mpy-tool.py``'s ``-march-flags`` command-line option to set this
-value when creating MPY files.
+This is currently used to store which RISC-V processor extensions the MPY file
+needs to operate correctly besides I, M, C, and Zicsr.  Different flavours of
+ArmV7 are identified by their native architecture number, but reusing that
+mechanism would complicate things for RV32 and RV64.
+
+MPY files targeting RV32 or RV64 that do not need any particular processor
+extensions do not need to provide a flags integer (along with setting the
+appropriate bit in the header).  The lack of a flags value for RV32 and RV64
+MPY files is used to indicate that no specific extensions are needed, and saves
+one byte in the final output binary.
+
+See also the ``-march-flags`` command-line option in both ``mpy-tool.py`` and
+``mpy-cross`` to set this value when creating MPY files.
 
 The global qstr and constant tables
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~