porting guidelines: fix bad link

Change the link to point to the correct page when referring to
configuration and compile time flags. In the same section fix the bullet
list so it renders correctly.

Fixes: #142
Signed-off-by: Joakim Bech <[email protected]>
Reviewed-by: Jerome Forissier <[email protected]>
Reviewed-by: Jens Wiklander <[email protected]>

architecture/porting_guidelines.rst

@@ -53,14 +53,17 @@ folder:
 **conf.mk**
 
 This is the device specific makefile where you define configurations unique to
-your platform. This mainly comprises two things: - OP-TEE configuration
-variables (``CFG_``), which may be assigned values in two ways. ``CFG_FOO ?=
-bar`` should be used to provide a default value that may be modified at compile
-time. On the other hand, variables that must be set to some value and cannot be
-modified should be set by: ``$(call force,CFG_FOO,bar)``. - Compiler flags for
-the TEE core, the user mode libraries and the Trusted Applications, which may be
-added to macros used by the build system. Please see `Platform-specific
-configuration and flags`_ in the build system documentation.
+your platform. This mainly comprises two things:
+
+- OP-TEE configuration variables (``CFG_``), which may be assigned values in
+  two ways. ``CFG_FOO ?= bar`` should be used to provide a default value that
+  may be modified at compile time. On the other hand, variables that must be
+  set to some value and cannot be modified should be set by: ``$(call
+  force,CFG_FOO,bar)``.
+
+- Compiler flags for the TEE core, the user mode libraries and the Trusted
+  Applications, which may be added to macros used by the build system. Please
+  see :ref:`configuration_and_flags` and similar sections on that page.
 
 It is recommended to use a existing platform configuration file as a starting
 point. For instance, `core/arch/arm/plat-hikey/conf.mk`_.
@@ -433,6 +436,5 @@ their products on. Refer also to :ref:`platform_documentation`
 .. _MAINTAINERS.md: https://github.com/OP-TEE/optee_os/blob/master/MAINTAINERS
 .. _OTP: https://en.wikipedia.org/wiki/Programmable_read-only_memory
 .. _plat_arm_security_setup: https://github.com/ARM-software/arm-trusted-firmware/search?utf8=%E2%9C%93&q=plat_arm_security_setup&type=
-.. _Platform-specific configuration and flags: build_system.md#platform-specific-configuration-and-flags
 .. _TZC-380: http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0431c/index.html
 .. _TZC-400: http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.100325_0001_02_en/index.html