Convert CONFIG_SPL_STACK to Kconfig

This converts the following to Kconfig:
   CONFIG_SPL_STACK

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/h8_homlet_v2_defconfig b/configs/h8_homlet_v2_defconfig
index 795de6f..8af0b3c 100644
--- a/configs/h8_homlet_v2_defconfig
+++ b/configs/h8_homlet_v2_defconfig
@@ -11,6 +11,7 @@
 CONFIG_AXP_GPIO=y
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
 CONFIG_CONSOLE_MUX=y
+CONFIG_SPL_STACK=0x8000
 CONFIG_SYS_PBSIZE=1024
 CONFIG_AXP_DLDO4_VOLT=3300
 CONFIG_USB_EHCI_HCD=y