Initial work on Ubuntu port of Mongoose OS

This PR yields mostly running code on Ubuntu LTS amd64.

docker image mgos/ubuntu-build
Makefile.build written from scratch
- build/fs/ is not yet assembled.
Some libraries are compromised:
- mongoose is copied in (no libmongoose-ubuntu.a yet).
- Application does not include rpc-* libraries.
- mbedtls does not include atca
- mbedtls should be provided (libmbedtls-ubuntu.a) upstream
HAL stubbed out, and partially implemented
- some lower level calls (network, rlock/lock, wdt, memory, etc) are complete.
- lots of work to be done on the remaining HAL files.
No filesystem support yet. conf0.json is read from CWD.
- Will initially run from build/fs/ filesystem
- Should ultimately run in a chroot() or a fs.bin (mmap'd) block device.

This is now to the point that it compiles and runs empty.git application with a 1000ms callback timer.

$ build/bin/empty 
mongoose_init        Mongoose OS 201901172117 (20190117-211718/2.10.2-25-gaf42cabb-dirty-master)
mongoose_init        empty 1.0 (20190117-211718/2.10.0-dirty-master-g01d3f5b73)
mongoose_init        CPU: 1742 MHz, heap: 16799997952 total, 2030559232 free
mongoose_init        Network: ip=194.126.235.37 netmask=255.255.255.240 gateway=194.126.235.33
mgos_hw_timers_init  Not implemented yet
mgos_vfs_common_init Not implemented yet
mgos_core_fs_init    Not implemented yet
mgos_gpio_hal_init   Not implemented yet
mgos_conf_parse_cb   Extra key: [debug.udp_log_addr]
mgos_conf_parse_cb   Extra key: [debug.mbedtls_level]
mgos_sys_config_init MAC: 03094340DE72
mgos_sys_config_init WDT: 30 seconds
mgos_init            Init done, RAM: 3915096064 total, 2030559232 free, 0 min free
app_cb               counter=1
app_cb               counter=2
app_cb               counter=3
app_cb               counter=4
app_cb               counter=5
app_cb               counter=6
app_cb               counter=7
app_cb               counter=8

cesanta / mongoose-os

Initial work on Ubuntu port of Mongoose OS #475