* 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
* All components which won't build (yet) on Linux are excluded.
This enables switching to Linux in an application without
explicitly setting COMPONENTS to main in the main
CMakeLists.txt.
* ESP Timer provides headers for Linux now
* automatically disabling LWIP in Kconfig if it is not available
doc(linux): brought section
"Component Linux/Mock Support Overview" up to date
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
* Philosophical: "explicit is better than implicit".
* Practical: Allows useful errors if invalid directories given in components as the defaults aren't
always used. Also trims the -I path from a number of components that have no actual include
directory.
* Simplifies knowing which components will be header-only and which won't
