Convert CONFIG_SYS_MAX_FLASH_SECT to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_MAX_FLASH_SECT

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/ethernut5_defconfig b/configs/ethernut5_defconfig
index 796b717..6725e3a 100644
--- a/configs/ethernut5_defconfig
+++ b/configs/ethernut5_defconfig
@@ -71,6 +71,7 @@
 CONFIG_GENERIC_ATMEL_MCI=y
 CONFIG_MTD=y
 CONFIG_MTD_NOR_FLASH=y
+CONFIG_SYS_MAX_FLASH_SECT=32
 CONFIG_MTD_RAW_NAND=y
 # CONFIG_SYS_NAND_USE_FLASH_BBT is not set
 CONFIG_NAND_ATMEL=y