Convert CONFIG_SYS_BOOTM_LEN to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_BOOTM_LEN

As part of this, rework error handling in boot/bootm.c so that we pass
the buffer size to handle_decomp_error as CONFIG_SYS_BOOTM_LEN will not
be available to host tools but we do know the size that we passed to
malloc().

Cc: Soeren Moch <smoch@web.de>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/stm32mp13_defconfig b/configs/stm32mp13_defconfig
index b5dcec7..f5b6262 100644
--- a/configs/stm32mp13_defconfig
+++ b/configs/stm32mp13_defconfig
@@ -18,6 +18,7 @@
 CONFIG_BOOTDELAY=1
 CONFIG_BOOTCOMMAND="run bootcmd_stm32mp"
 CONFIG_SYS_PROMPT="STM32MP> "
+CONFIG_SYS_BOOTM_LEN=0x2000000
 CONFIG_CMD_ADTIMG=y
 CONFIG_CMD_ERASEENV=y
 CONFIG_CMD_MEMINFO=y