package/micropython: fix build with gcc 4.8

package/micropython: fix build with gcc 4.8

micropython uses -Wfloat-conversion since version 1.13 and
https://github.com/micropython/micropython/commit/30840ebc9925bb8ef025dbc2d5982b1bfeb75f1b

This will result in the following build failure with gcc 4.8:
arm-none-linux-gnueabi-gcc: error: unrecognized command line option '-Wfloat-conversion'

To avoid this build failure, set CWARN to an empty value moreover, while
at it, move some duplicated variables under MICROPYTHON_MAKE_OPTS

Fixes:
 - http://autobuild.buildroot.org/results/6fe5a5ab91a5d235147e74461bb165ec2cfdc967

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>