Microwatt may have firmware that places data in r3, which was used to
detect microwatt vs powernv. This breaks the existing probing of the UART
type in this powerpc port.
Instead build only the appropriate UART into the firmware, selected by
passing the option UART=potato or UART=lpc_serial to the Makefile.
A future enhancement would be to parse the device tree and configure
MicroPython based on the settings.
The linker script was included in the "$^" inputs, causing the build to
fail:
LINK build/firmware.elf
powerpc64le-linux-gnu-ld: error: linker script file 'powerpc.lds' appears multiple times
As a fix the linker script is left as a dependency of the elf, but only the
object files are linked.
Add -Wdouble-promotion and -Wfloat-conversion for most ports to ban out
implicit floating point conversions, and add extra Travis builds using
MICROPY_FLOAT_IMPL_FLOAT to uncover warnings which weren't found
previously. For the unix port -Wsign-comparison is added as well but only
there since only clang supports this but gcc doesn't.
