bootm: Add a Kconfig option for bootm functionality

Create a separate Kconfig option which enables the bootm logic,
separate from the 'bootm' command. This will eventually allow booting
without CMDLINE enabled.

Update boards which disable CMD_BOOTM to disable BOOTM instead, since
CMD_BOOTM now depends on BOOTM

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/zynq_cse_nand_defconfig b/configs/zynq_cse_nand_defconfig
index f7b7581..521805c 100644
--- a/configs/zynq_cse_nand_defconfig
+++ b/configs/zynq_cse_nand_defconfig
@@ -19,6 +19,7 @@
 CONFIG_REMAKE_ELF=y
 CONFIG_SYS_CUSTOM_LDSCRIPT=y
 CONFIG_SYS_LDSCRIPT="arch/arm/mach-zynq/u-boot.lds"
+# CONFIG_BOOTM is not set
 # CONFIG_AUTOBOOT is not set
 CONFIG_USE_PREBOOT=y
 # CONFIG_DISPLAY_CPUINFO is not set
@@ -43,7 +44,6 @@
 # CONFIG_CMD_BDI is not set
 # CONFIG_CMD_CONSOLE is not set
 # CONFIG_CMD_BOOTD is not set
-# CONFIG_CMD_BOOTM is not set
 # CONFIG_CMD_ELF is not set
 # CONFIG_CMD_FDT is not set
 # CONFIG_CMD_GO is not set