Bump skiboot to 5.3.0 for all platforms

Post op-build v1.10, we're bringing all platforms up to latest skiboot.

Effectively, 5.2.x won't be maintained unless somebody steps up to do it.

See the skiboot release notes for changes:
https://github.com/open-power/skiboot/blob/skiboot-5.3.0/doc/release-notes/skiboot-5.3.0.txt
https://github.com/open-power/skiboot/blob/skiboot-5.3.0/doc/release-notes/skiboot-5.3.0-rc1.txt
https://github.com/open-power/skiboot/blob/skiboot-5.3.0/doc/release-notes/skiboot-5.3.0-rc2.txt

Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>
diff --git a/openpower/configs/openpower_p9_mambo_defconfig b/openpower/configs/openpower_p9_mambo_defconfig
index f7672a4..7dfbf8b 100644
--- a/openpower/configs/openpower_p9_mambo_defconfig
+++ b/openpower/configs/openpower_p9_mambo_defconfig
@@ -41,6 +41,4 @@
 # BR2_PACKAGE_OPENPOWER_PNOR is not set
 BR2_OPENPOWER_CONFIG_NAME="openpower-mambo-p9"
 BR2_PACKAGE_PETITBOOT=y
-# BR2_PACKAGE_OCC is not set
-BR2_SKIBOOT_CUSTOM_VERSION=y
-BR2_SKIBOOT_CUSTOM_VERSION_VALUE="skiboot-5.3.0"
+# BR2_PACKAGE_OCC is not set
\ No newline at end of file