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/microchip_mpfs_icicle_defconfig b/configs/microchip_mpfs_icicle_defconfig
index 462c8b7..cf09353 100644
--- a/configs/microchip_mpfs_icicle_defconfig
+++ b/configs/microchip_mpfs_icicle_defconfig
@@ -16,6 +16,7 @@
 CONFIG_SYS_PROMPT="RISC-V # "
 CONFIG_SYS_CBSIZE=256
 CONFIG_SYS_PBSIZE=282
+CONFIG_SYS_BOOTM_LEN=0x4000000
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_BOOTP_SEND_HOSTNAME=y
 CONFIG_DM_MTD=y