cpu/cc2538: remove obsolete periph file guard

boards/common/atmega: gracefully handle CKDIV8 fuse

gnrc/netif/hdr.h: Optimized structure layout

Reordered struct members to not waste memory due to padding.

Before:
``` C
typedef struct {
    uint8_t src_l2addr_len;
    uint8_t dst_l2addr_len;
    kernel_pid_t if_pid;    // <-- 16 bit, is aligned to 16 bit
    uint8_t flags;
    uint8_t __padding_byte; // <-- Inserted to fulfill padding requirements
    int16_t rssi;           // <-- 16 bit, is NOT aligned to 16 bit
    uint8_t lqi;
    uint8_t __padding_byte2;// <-- Inserted to fulfill padding requirements
} gnrc_netif_hdr_t;
```

Now:
``` C
typedef struct {
    uint8_t src_l2addr_len;
    uint8_t dst_l2addr_len;
    kernel_pid_t if_pid;    // <-- 16 bit, is aligned to 16 bit
    uint8_t flags;
    uint8_t lqi;
    int16_t rssi;           // <-- 16 bit, is aligned to 16 bit
} gnrc_netif_hdr_t;
```

When build for the `bluepill` board, the new layout reduces the size by 2 bytes.

makefiles/info.inc.mk: add a info-debug-variable-% target

tests: unittests: disable for new efm32 boards.

cpu/cortexm: explicitly set -march=armv6s-m for crotexm0(plus)

drivers/io1_xplained: improve driver and add support for the sdcard and light sensor

drivers: add driver for FXOS8700 3-axis accelerometer/magnetometer

enc28j60: fix compilation with l2 stats enabled

 cpu/nrf5x: define nrfmin feature in nrf5x_common

tests/mutex_unlock_and_sleep: increase timeout

sys/icmpv6_hdr_print: replace PRIu8 by PRIu16

Works around a regression in gcc 8.1.
See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85606.

cortexm: const ISR vectors

tests/trickle: initialize prev_now in main

tests/pkg_cayenne_lpp: fix inconsistent buffer values across architectures (native, boards)

dist/testbed-support: use new cli-tools command names

- generated payloads can differ between architectures due to floating precision
- reworked how buffer content is tested in the application
- second test value depends on architecture (native and other platforms)

boards/iotlab: fix saul gpio configuration

release notes 2018.04 typo fix

Having `prev_now` initialized to 0 breaks tests on `arduino-zero` and
`arduino-mega2560` as `xtimer_now_usec` is way bigger (72k on `arduino-zero`).

Issue found in:
* #9052 and proposed fix by ZetaR60
* https://github.com/RIOT-OS/Release-Specs/issues/62#issuecomment-387421737

PRIu8 is not supported on samr21-xpro:

    type:   hu  code:   hu

After fix

    type: 133  code:   0

Found by running 'tests/gnrc_netif'

add 2018.04 release notes

crypto/ccm: fix input_len checking with length_encoding

examples/gnrc_border_router: use RIOTTOOLS variable

Follow up to #9067 and part of #8821

Makefile.include: use RIOTTOOLS variable for included files

tests/libfixmath_unittests: fix for iotlab-m3 and samr21-xpro

cpu: ldscript memory attribute corrections

cpu/stm32f4: add support for STM32F423 line