* Users can now use libbsd string.h and sys/cdefs.h functionality
(e.g., strlcpy, containerof) on Linux by just including
string.h or sys/cdefs.h. In other words, the includes are the same
on the Linux target as well as on chips targets (ESP32, etc.).
* libbsd linking is done by the linux component (belongs to common
components) now instead of handling it separately in each component
linux/lwip: Wrap some IO posix functions
* to workaourd the FreeRTOS EINTR issue (when building without lwip)
* to correctly choose the sub-system based on fd (when building with
lwip) -- passing control to either linux/system or to lwip
This commit also addapts tapio-if to provide DHCP client by default and
configurable settings for static IP
Do not include bootloader in flash target when secure boot is enabled.
Emit signing warning on all cases where signed apps are enabled (secure
boot and signed images)
Follow convention of capital letters for SECURE_BOOT_SIGNING_KEY
variable, since it is
relevant to other components, not just bootloader.
Pass signing key and verification key via config, not requiring
bootloader to know parent app dir.
Misc. variables name corrections
