hashes/sha256: don't call memcpy if len==0

fmt: fix fmt_s32_dec() and fmt_s64_dec() sign bit handling

cpu/native/periph/timer: fix type conversion

tests/build_system_utils: fix TESTS name collision

pkg/jerryscript: namespace BUILD_DIR variable

gnrc/pktbuf: avoid memcpy if size <= 0

TESTS is used to find the tests files for the application but was used
to list test targets. Rename to COMPILE_TESTS as already done in some of
my other applications.

BUILD_DIR was introduced but the variable was already used by
`pkg/jerryscript` so there is a name collision.
Namespace it to prevent issues.

cpu/cortexm_common: Update ARM CMSIS to V5.4.0

 gnrc_netif: centralize device-type-specific functions

The function to infer the link-layer address length from the length of
a S/TLLAO is very dependent on the IPv6 over X specification and thus
should be grouped with the other IP over X functions.

"val = -val" causes UB for INTMIN, thus explicitly cast to unsigned.

make: introduce common BUILD_DIR

asymcute: check for minimum packet length early

pkg/lua: Fix warnings on discarded qualifiers.

 drivers/i2c: Fix i2c api issues

Without this patch _len_get reads one byte beyond the con->rxbuf
if the incoming packet consists only of the byte 0x01.

Do normal assignment instead of memcpy

gnrc_sixlowpan_frag: drop received duplicates

Signed-off-by: Dylan Laduranty <dylan.laduranty@mesotic.com>

Otherwise duplicate fragments are added to the number of already
received datagram bytes in L153 leading to possible race-conditions.

boards/iotlab-m3: disable DMA with SPI

riotboot: add multislot support

Signed-off-by: Dylan Laduranty <dylan.laduranty@mesotic.com>

shell: sc_icmpv6_echo: release payload when pktbuf full

There was a cast in a macro that calculated offsets. Since it was
only doing a pointer difference, the cast was inocuous, but I fix
it anyways.

Upstream lua has many instances of discarding cont qualifiers through
explicit casting. This disables the warnigns for the package so that
they don't show up when one enables -Wno-cast-qual globally.

Otherwise it gets never released

Co-Authored-By: Gunar Schorcht <gunar@schorcht.net>

.travis.yml: provide base branch to static tests