make: fix various compile errors with Wextra

        pkg, nordic_softdevice_ble: disable CFLAGS to omit compiler error
        sys, pm_layered: fix casting nonscalar to the same type
        cpu, stm32_common: fix type-limits, remove always true assert
        cpu, stm32f4: fix pointer arithmetic in periph/i2c
        drivers, at86rf2xx: fix type-limits where condition always true
        saul, gpio: fix if no gpio configured for saul
        cpu, saml21: add frequency check to periph/timer
        driver, cc110x: fix unused param and type-limts errors
        boards, wsn430-common: fix old-style-declaration
        make: fix old style definition
        drivers, sdcard_spi: fix old style typedef
        driver, at30tse: remove unnecessary check
        driver, nrf24: fix type-limit
        driver, pn532: change buffer from char to uint8_t
        tests/driver_sdcard: fix type limits
        boards, feather-m0: add missing field inits
        driver, tcs37727: fix type limits
        pkg, emb6: disable some compiler warnings
        tests/emb6: disable some compiler warings
        pkg, openthread: fix sign compare and unused params
        tests/trickle: fix struct init
        tests/pthread_cooperation: fix type limits
        board, mips-malta: remove feature periph_uart
        shell: fix var size for netif command
        gnrc, netif: fix sign-compare
        gnrc, nib: fix sign-compare
        shell: fix output in netif command
        posix: fix type-limits in pthread_cond