Convert CONFIG_SYS_NAND_U_BOOT_LOCATIONS et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_NAND_U_BOOT_LOCATIONS
   CONFIG_SYS_NAND_U_BOOT_OFFS

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/axm_defconfig b/configs/axm_defconfig
index 520e2ff..9719301 100644
--- a/configs/axm_defconfig
+++ b/configs/axm_defconfig
@@ -77,6 +77,8 @@
 CONFIG_SYS_NAND_PAGE_COUNT=0x40
 CONFIG_SYS_NAND_PAGE_SIZE=0x800
 CONFIG_SYS_NAND_OOBSIZE=0x40
+CONFIG_SYS_NAND_U_BOOT_LOCATIONS=y
+CONFIG_SYS_NAND_U_BOOT_OFFS=0x20000
 CONFIG_DM_SPI_FLASH=y
 CONFIG_SPI_FLASH_STMICRO=y
 CONFIG_PHYLIB=y