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/xilinx_versal_mini_emmc1_defconfig b/configs/xilinx_versal_mini_emmc1_defconfig
index 747f20d..f649200 100644
--- a/configs/xilinx_versal_mini_emmc1_defconfig
+++ b/configs/xilinx_versal_mini_emmc1_defconfig
@@ -15,6 +15,7 @@
 CONFIG_SYS_LOAD_ADDR=0x8000000
 # CONFIG_EXPERT is not set
 CONFIG_REMAKE_ELF=y
+# CONFIG_BOOTM is not set
 # CONFIG_AUTOBOOT is not set
 CONFIG_SYS_CONSOLE_INFO_QUIET=y
 # CONFIG_DISPLAY_CPUINFO is not set
@@ -31,7 +32,6 @@
 # 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_BOOTI is not set
 # CONFIG_CMD_ELF is not set
 # CONFIG_CMD_FDT is not set