[klibc] [klibc:master] README.klibc: Document most of the build options

[klibc-bot for Ben Hutchings]

Commit-ID:  4876d484577d8a4d9ff7de437fcfa5cba7a7e748
Gitweb:     http://git.kernel.org/?p=libs/klibc/klibc.git;a=commit;h=4876d484577d8a4d9ff7de437fcfa5cba7a7e748
Author:     Ben Hutchings <ben at decadent.org.uk>
AuthorDate: Fri, 1 Feb 2019 18:04:52 +0100
Committer:  Ben Hutchings <ben at decadent.org.uk>
CommitDate: Sat, 2 Feb 2019 00:13:32 +0100

[klibc] README.klibc: Document most of the build options

* Explicitly say where to set the cross-compilation variables
* Explain how to do an out-of-tree build
* Document all the configuration variables in defconfig

Signed-off-by: Ben Hutchings <ben at decadent.org.uk>

---
 usr/klibc/README.klibc | 47 ++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 46 insertions(+), 1 deletion(-)

diff --git a/usr/klibc/README.klibc b/usr/klibc/README.klibc
index ea10c55f..032d3959 100644
--- a/usr/klibc/README.klibc
+++ b/usr/klibc/README.klibc
@@ -22,7 +22,7 @@ Cross-compiling
 
 If you're cross-compiling, you need to set KLIBCARCH to the
 appropriate architecture, and set CROSS_COMPILE to your toolchain
-prefix.
+prefix, on the "make" command line
 
 IMPORTANT: if you're on a 64-bit machine with a 32-bit userland
 (ia64, mips64, ppc64 sparc64, s390x or x86_64), and you want to
@@ -36,6 +36,51 @@ If you are on ARM, and want to build a thumb version of the library
 (this is supported), change OPTFLAGS in arch/arm/MCONFIG to build
 thumb code.
 
+Out-of-tree builds
+------------------
+
+It is possible to use a separate build directory so that build
+products are not created in source directories.  To do this,
+create the build directory and in that directory run:
+    make -f <source-dir>/Makefile KBUILD_SRC=<source-dir>
+
+Build configuration
+-------------------
+
+The build configuration is defined in a ".config" file in the
+build directory.  If this file does not already exist, it is
+created as a copy of the "defconfig" source file.
+
+The configuration variables are:
+
+* CONFIG_KLIBC_ERRLIST (bool): Include standard error strings for
+  strerror().  If disabled, strerror() returns the error number as a
+  string.
+* CONFIG_KLIBC_ZLIB (bool): Include zlib decompression functions.  If
+  disabled, kinit can only load uncompressed ramdisk images.
+* CONFIG_KLIBC_ZIP (bool): Include compression support in the gzip
+  command.
+* CONFIG_DEBUG_INFO (bool): Install all executables and the shared
+  library with debug information and build IDs included.  If
+  disabled, the executables and shared library are stripped when
+  installed.
+
+For arm only:
+
+* CONFIG_KLIBC_THUMB (bool): Compile all code to Thumb instructions,
+  which should reduce code size slightly.  If disabled, regular ARM
+  instructions are used.
+* CONFIG_AEABI (bool): Compile ARM code to use the ARM EABI and the
+  "new" system call ABI.
+
+If both CONFIG_KLIBC_THUMB and CONFIG_AEABI are disabled, the GNU APCS
+and the old system call ABI are used.
+
+For i386 only:
+
+* CONFIG_REGPARM (bool): Optimise function calls by passing the first
+  3 function parameters in registers.
+
 Building without kernel source
 ------------------------------