Add convenience header: os/mynewt.h

[ccollins476ad]

This new header (os/mynewt.h) includes the following headers:

• syscfg/syscfg.h
• sysinit/sysinit.h
• sysflash/sysflash.h
• os/os.h
• defs/error.h

This header is intended to be included before any other non-system headers. The purpose of this header is to make an application developer's job easier, and simplify the "getting started" process.

I think a good rule of thumb is: always include os/mynewt.h and never directly include any of the headers listed above.

I put this header in the kernel/os package because, in practice, every other package has access to this one. I am happy to move it somewhere else if there are alternative ideas.

I was tempted to include bsp/bsp.h in this header, but I decided it is better if Mynewt doesn't make any assumptions about which files the BSP exports.

I tested this PR with newt test all and by successfully building the following targets (most target definitions can be inferred from their names):

blecent-nrf51dk
blecent-nrf52dk
blehci-nrf51dk
blehci-nrf52dk
blehr-nrf52dk
bleprph-ada_feather_nrf52
bleprph-arduino_primo_nrf52
bleprph-bbc_microbit
bleprph-nrf51dk
bleprph-nrf51dk-16kbram
bleprph-nrf52-thingy
bleprph-nrf52dk
bleprph_oic-nrf52dk
blesplit-bbc_microbit
blesplit-nrf51dk
blesplit-nrf52dk
bletiny-bmd300
bletiny-nrf52dk
bletiny-nrf52dk-noleg
bletiny-nrf52dk-nosc
bleuart-nrf52dk
blinky-ada_feather_nrf52
blinky-apollo2_evb
blinky-arduino_primo_nrf52
blinky-bbc_microbit
blinky-ble400
blinky-bmd200
blinky-bmd300eval
blinky-calliope_mini
blinky-frdm-k64f
blinky-native
blinky-nina-b1
blinky-nrf51-arduino_101
blinky-nrf51-blenano
blinky-nrf51dk
blinky-nrf51dk-16kbram
blinky-nrf52-thingy
blinky-nrf52840pdk
blinky-nrf52dk
blinky-nucleo-f303k8
blinky-nucleo-f303re
blinky-nucleo-f401re
blinky-nucleo-f413re
blinky-nucleo-f413zh
blinky-olimex-p103
blinky-olimex_stm32-e407_devboard
blinky-puckjs
blinky-rb-blend2
blinky-rb-nano2
blinky-ruuvi_tag_revb2
blinky-stm32f3discovery
blinky-stm32f429discovery
blinky-stm32f4discovery
blinky-stm32f767-nucleo
blinky-stm32f7discovery
blinky-stm32l152discovery
blinky-telee02
blinky-usbmkw41z
blinky-vbluno51
blinky-vbluno52
boot-101
boot-apollo2_evb
boot-arduino_primo_nrf52
boot-bbc_microbit
boot-bmd300eval
boot-frdm-k64f
boot-nrf51dk
boot-nrf51dk-16kbram
boot-nrf52dk
boot-nucleo-f413re
boot-nucleo413zh
boot-olimex_stm32-e407_devboard
boot-rb-nano2
boot-serial-nrf52dk
boot-telee02
bsncent-nrf52dk
bsnprph-nrf52dk
btshell-native
btshell-nrf52dk
iptest_f7
my_blinky_sim
ocf_sample-nrf52dk
ocf_sample-sim
sensors_test-nrf52dk
slinky-apollo2_evb
slinky-nrf51dk
slinky-nrf51dk-16kbram
slinky-nrf52dk
slinky-olimex_stm32-e407_devboard
slinky-sim
slinky_oic-nrf52dk
slinky_oic-sim
spitest-m-apollo2_evb
spitest-nrf52dk
spitest-nucleo-f413re
spitest-nucleo-f413zh
spitest-s-nrf52dk
split-nrf51dk
splitty-nrf52dk
testbench-nrf51dk
testbench-nrf52dk
timtest-nrf52dk

[vrahane]

Looks good.

[utzig]

Too complicated to review thoroughly but looks good in a glance!