lbmk: unified PWD handling (work directory)

instead of running pwd all the time, run it once in lib.sh,
and export PWD.

for lbmk-specific use of PWD, use xbmkpwd, which contains
the value of PWD as was set by the pwd utility in lib.sh.

many parts of lbmk rely on pwd, and it *must* be correct.
this change adds basic error handling, since pwd can in
fact return errors in some cases.

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

script/trees

@@ -174,7 +174,7 @@ configure_project()
 	[ -z "$btype" ] || [ "${mode%config}" = "$mode" ] || return 1
 	[ -z "$mode" ] && build_dependencies
 
-	mdir="`pwd`/config/submodule/$project"
+	mdir="$xbmkpwd/config/submodule/$project"
 	[ -n "$tree" ] && mdir="$mdir/$tree"
 	[ -f "CHANGELOG" ] || check_project_hashes
 
@@ -245,7 +245,7 @@ check_cross_compiler()
 
 		x_ ./mk -f coreboot "${cbdir#src/coreboot/}"
 
-		export PATH="`pwd`/$cbdir/util/crossgcc/xgcc/bin:$PATH"
+		export PATH="$xbmkpwd/$cbdir/util/crossgcc/xgcc/bin:$PATH"
 		export CROSS_COMPILE="${xarch% *}-"
 		[ -n "$xlang" ] && export BUILD_LANGUAGES="$xlang"