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/stm32mp15_defconfig b/configs/stm32mp15_defconfig
index 48185d3..3452403 100644
--- a/configs/stm32mp15_defconfig
+++ b/configs/stm32mp15_defconfig
@@ -23,6 +23,7 @@
 CONFIG_FDT_SIMPLEFB=y
 CONFIG_SYS_PROMPT="STM32MP> "
 CONFIG_SYS_PBSIZE=1050
+CONFIG_SYS_BOOTM_LEN=0x2000000
 CONFIG_CMD_ADTIMG=y
 CONFIG_CMD_ERASEENV=y
 CONFIG_CMD_NVEDIT_EFI=y