musl: two-stage gcc simplifications

After switching to a two stage gcc solution, there is no longer a need
to do weird things in the musl build, with certain things being done
twice (MUSL_CONFIGURE_CALL). Now the MUSL_CONFIGURE_CMDS variable only
does the configuration, and the MUSL_BUILD_CMDS only does the build,
as it should be.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>