Before this commit, weak symbols were not overridden by non-weak symbols
found in archive libraries when linking with recent versions of
binutils.  As stated in the System V ABI, "the link editor does not
extract archive members to resolve undefined weak symbols".

This commit changes all Makefiles to use partial linking (ld -r) instead
of creating library archives, which forces all symbols to participate in
linking, allowing non-weak symbols to override weak symbols as intended.
This approach is also used by Linux, from which the gmake function
cmd_link_o_target (defined in config.mk and used in all Makefiles) is
inspired.

The name of each former library archive is preserved except for
extensions which change from ".a" to ".o".  This commit updates
references accordingly where needed, in particular in some linker
scripts.

This commit reveals board configurations that exclude some features but
include source files that depend these disabled features in the build,
resulting in undefined symbols.  Known such cases include:
- disabling CMD_NET but not CMD_NFS;
- enabling CONFIG_OF_LIBFDT but not CONFIG_QE.

Signed-off-by: Sebastien Carlier <sebastien.carlier@gmail.com>

This fixes a build breakage with CONFIG_API enabled, which appeared after
the recent changes in the U-Boot build system.

Signed-off-by: Rafal Jaworowski <raj@semihalf.com>

This is an API for external (standalone) applications running on top of
U-Boot, and is meant to be more extensible and robust than the existing
jumptable mechanism. It is similar to UNIX syscall approach. See api/README
for more details.

Included is the demo application using this new framework (api_examples).

Please note this is still an experimental feature, and is turned off by
default.

Signed-off-by: Rafal Jaworowski <raj@semihalf.com>