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/git/pico-serprog

@@ -1,7 +0,0 @@
-{pico-serprog}{
-	rev: e75e3a20e63269a5e3189bc2e49a6a81d45a636a
-	loc: pico-serprog
-	url: https://codeberg.org/libreboot/pico-serprog
-	bkup_url: https://git.disroot.org/libreboot/pico-serprog
-	depend: pico-sdk
-}

config/git/pico-serprog/pkg.cfg

@@ -0,0 +1,4 @@
+rev="e75e3a20e63269a5e3189bc2e49a6a81d45a636a"
+url="https://codeberg.org/libreboot/pico-serprog"
+bkup_url="https://git.disroot.org/libreboot/pico-serprog"
+depend="pico-sdk"