[PATCH v8 7/7] docs: Update user_manual.md

["Andreas J. Reichel" <andreas.reichel.ext@siemens.com>]

From: Andreas Reichel <andreas.reichel.ext@siemens.com>

Explain DISTRO_BOOTSTRAP_KEYS and THIRD_PARTY_APT_KEYS.

Signed-off-by: Andreas Reichel <andreas.reichel.ext@siemens.com>
---
 doc/user_manual.md | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/doc/user_manual.md b/doc/user_manual.md
index 6fe4d83..8db636b 100644
--- a/doc/user_manual.md
+++ b/doc/user_manual.md
@@ -374,14 +374,17 @@ Isar can generate various images types for specific machine. The type of the ima
 The distro is defined by the set of the following variables:
 
  - `DISTRO_APT_SOURCES` - List of apt source files
- - `DISTRO_APT_KEYS` - List of gpg key URIs used to verify apt repos
+ - `DISTRO_BOOTSTRAP_KEYS` - List of gpg key URIs used to verify the apt repo used during bootstrapping
+ - `THIRD_PARTY_APT_KEYS` - List of gpg key URIs used to verify apt repos for apt installation after bootstrapping
  - `DISTRO_APT_PREFERENCES` - List of apt preference files
  - `DISTRO_KERNELS` - List of supported kernel suffixes
 
+The first entry of the apt sources is used for bootstrapping.
+
 Below is an example for Raspbian Jessie:
 ```
 DISTRO_APT_SOURCES += "conf/distro/raspbian-jessie.list"
-DISTRO_APT_KEYS += "https://archive.raspbian.org/raspbian.public.key;sha256sum=ca59cd4f2bcbc3a1d41ba6815a02a8dc5c175467a59bd87edeac458f4a5345de"
+DISTRO_BOOTSTRAP_KEYS += "https://archive.raspbian.org/raspbian.public.key;sha256sum=ca59cd4f2bcbc3a1d41ba6815a02a8dc5c175467a59bd87edeac458f4a5345de"
 DISTRO_CONFIG_SCRIPT?= "raspbian-configscript.sh"
 DISTRO_KERNELS ?= "rpi rpi2 rpi-rpfv rpi2-rpfv"
 ```
-- 
2.21.0