kpatch-build: building OOT livepatch modules for kernels built from source

Ability to build out-of-tree modules for non-host kernel is currently
missing. This commit introduces that ability for any kernel built
from source, including cross-compiled ones. The workaround checks for
existence of /lib/modules/$ARCHVERSION/build, and if nonexistent,
assumes the kernel source directory to be a build directory.

Author: omatiusha

kpatch-build/kpatch-build

@@ -1050,9 +1050,12 @@ ERROR=0
 
 # Prepare OOT module symvers file
 if [[ -n "$OOT_MODULE" ]]; then
-    BUILDDIR="/lib/modules/$ARCHVERSION/build/"
-    cp -f "$SRCDIR/Module.symvers" "$TEMPDIR/Module.symvers" || die
-    awk '{ print $1 "\t" $2 "\t" $3 "\t" $4}' "${BUILDDIR}/Module.symvers" >> "$TEMPDIR/Module.symvers"
+	cp -f "$SRCDIR/Module.symvers" "$TEMPDIR/Module.symvers" || die
+	# merge host symbols only if we do compile for host kernel
+	if [[ -e "/lib/modules/$ARCHVERSION/build/" ]]; then
+		BUILDDIR="/lib/modules/$ARCHVERSION/build/"
+		awk '{ print $1 "\t" $2 "\t" $3 "\t" $4}' "${BUILDDIR}/Module.symvers" >> "$TEMPDIR/Module.symvers"
+	fi
 fi
 
 for i in $FILES; do
@@ -1164,7 +1167,12 @@ cd "$TEMPDIR/patch" || die
 if [[ -z "$OOT_MODULE" ]]; then
 	KPATCH_BUILD="$SRCDIR"
 else
-	KPATCH_BUILD="/lib/modules/$ARCHVERSION/build"
+	if [[ ! -e "/lib/modules/$ARCHVERSION/build" ]]; then
+		#assume we build for kernel built from source
+		KPATCH_BUILD="$(dirname "$CONFIGFILE")"
+	else 
+		KPATCH_BUILD="/lib/modules/$ARCHVERSION/build"
+	fi
 fi
 
 # We no longer need kpatch-cc