lib.sh: more unified config handling

replace it with logic that simply uses "." to load
files directly. for this, "vcfg" is added as a variable
in coreboot target.cfg files, referring to a directory
in config/vendor/ containing a file named pkg.cfg, and
this file then contains the same variables as the
erstwhile config/vendor/sources

config/git files are now directories, also containing
pkg.cfg files each with the same variables as before,
such as repository link and commit hash

this change results in a noticeable reduction in code
complexity within the build system.

unified reading of config files: new function setcfg()
added to lib.sh

setcfg checks if a config exists. if a 2nd argument is
passed, it is used as a return value for eval, otherwise
a string calling err is passed. setcfg output is passed
through eval, to set strings based on config; eval must
be used, so that the variables are set within the same
scope, otherwise they'd be set within setcfg which could
lead to some whacky results.

there's still a bit more more to do, but this single change
results in a substantial reduction in code complexity.

Signed-off-by: Leah Rowe <leah@libreboot.org>

config/coreboot/x230_12mb/target.cfg

@@ -4,3 +4,4 @@ payload_seabios="y"
 payload_seabios_withgrub="y"
 payload_memtest="y"
 grub_scan_disk="ahci"
+vcfg="ivybridge"